It's hard to tell as all capital ships seem to have varying sizes.

I've calculated that the Radiance Battleship is about 4.5 km in length.

The Eradica is about four times as tall as the Radiance is long which puts it at almost 20km height. If all Titans have approximately the same size, they're all between 15 to 25km in their longest dimension. That would make the Ragnarov a 20km long rail cannon.

But as I've said, all capital ships seem different. The Kol, though it should be of somewhat equivalent size to the Radiance, appears to be relatively small, like hardly over 1km in length. On the other hand, the Evacuator has entire cities built into it, it's probably massive.

I've just come to the arbitrary conclusion that frigates are between 500-1000m, cruiser between 1 and 2 km, capital ships 4 to 6 km and titans 15-25km.

I didn't get the size of the Radiance by comparing it to anything else, and the planets were obviously made smaller for gameplay purposes.

As GoaFan said, in the end it is irrelevant. The only purpose of knowing these things is to satisfy curiosity. You could go around measuring each ship from its textures, or try to compare them all to the colony frigate after estimating its size from the intro video, whatever floats your boat. I've chosen to believe as I explained before.

He probably looked at texture windows and scaled them to a person?

Here's a question ... based on assuming a window size, and the knowledge that each ship in game is not to scale with each other ...

how about a mod that truly scales each ship in game to it's proper scale? (planets aside)

I think it is obvious that for any size guess we have to ignore the planets, as they have been greatly reduced in scale for gameplay purposes. Instead we should only compare ship against each others. 

Imho with the use of the colony frigate seen in the intro one could make a halfway educated guess about the size of the other ships.

Also, the manual PDF file for Sins Rebellion comes with the information of how many people serve aboard the ships.

Here for example TEC:

Ship    Crew
Shriken Corvette    10
Stilat Corvette    10
Arcova Scout Frigate    75 (lots of sensor technicans probably)
Cobalt Light Frigate    150
Javelis LRM Frigate    200
Krosov Siege Frigate    175
Garda Flak Frigate    300
Protev Colony Frigate    500 (imaginable with the Intro)
Percheron Light Carrier    450
Hoshiko Robotics Cruiser    500
Cielo Command Cruiser    900
Kodiak Heavy Cruiser    475
Raloz Heavy Contructor    0 (automatic)
Ogrov Torpedo Cruiser    150
Neruda Envoy Cruiser    250
Kol Battleship    2.500
Sova Carrier    2.000
Akkan Battlecruiser    4.000
Dunov Battlecruiser    2.700
Corsev Battlecruiser    2.500
Marza Dreadnought    1.100
Ankylon Titan    5.000 - funny... the smallest Titan has more crew than the Ragnarov.)
Ragnarov Titan    3.500 (the Railgun and the ammunition depot probably take away a lot of space.)

Generally speaking, TEC military ships have probably somewhat cramped quarters.... the relative low technology level meaning that armor takes a lot of space and you need to have large ammunition depots for all that heavy firepower. Many windows are probably a leftover from its civilian service and I could imagine that under normal operation, there is not much crew sitting behind one. I think that while the interior of the ship is still quite large, most of the crew will concentrate in just a few areas deep inside the ship. Off duty they might have more room for a walk along the long corridors, but while on battle stations they probably mostly are in a well protected area of the ship.

That could explain why a much larger capital ship has just 5 times the crew of the colony frigate.

Another thing speaking for the very large size of the ships is the enormous ammount of punishment they can take. Even a corvette will withstand capital class firepower for a notable time period.... this is only possible with very large ammounts of armor that needs a lot of space. I would guess that the corvette crew is spread across the ship and that is still very cramped for 10 people should they meet off duty.

On a side note... if you take above values and compare them to the average battle..... the death count races quite quickly toward the 100000 count..... indeed quite a bloody war.

The Vorastra btw has 135000 crew.... the Kultorask 132000.... speak about superior technology and probably still cramped quarters.

How would you go about determining the size of the hangar or the strike craft? Not only am I certain that sins' SC are larger than today's aircraft thanks to all the extra systems they'd need like extra propulsion to maneuver in space as well as thicker hulls to remain airtight even when shot, life support, stronger and more advanced weapons, etc... but I'm also quite sure the SC models have been made larger relative to other ships for them to be visible. 

For example, there's no way all three squadrons of the Radiance would fit in its hangar.

WTF Garda 300???? That small ship is all about exterior guns and strong armor combo, there is surely no need of high crew numbers, I would say 50 would be more than enough.. What are they doing in there? Partying all the time? Maybe 2 men are needed for each gun, some mechanics, some doctors, some for controlling ship movement, some for engine management, maybe 1-2 for shields (if it's not full-automatic), and 250 for running around like crazy if their ship is hit

These are very high numbers, and not scale well.. Akkan has more crew than the Ragnarov.. TEC shouldn't put this high number of skilled men on these ships, considering they are about to get destroyed sooner and to be replaced fast, TEC is having brutal losses this way, although not as high as Vasari, but they don't really have a choice.. Btw Vorastra and the Kultorask is a safe heaven for civilians as they are likely never get destroyed

Sova 2000? Such small number, it would be drained so easily in a big battle where its strike craft get destroyed in no time..

Based on today's technology level, it would be credible for TEC to use remote control on more ships, or set them automatic, as I don't really see 0 crew ships but the USA has unmanned drones in it's army... Strange.

These are far from realistic numbers in my opinion, but sure we have nothing better to start from (except Protev, see below), as many things are scaled unnaturally to be visible or not bother gameplay much.

Maybe the Protev is the only way to start scaling BUT! I am pretty sure Protev is not scaled well for other ships in the game, as it would make capital ships smaller than some guys estimated before. So the only ship to be scaled well is the Protev, someone can estimate its attributes from the opening video counting with the average human height as a base for measurements (I know it would change in that time they are ahead of us, but that some 10 cm-s are not high differences ) The Akkan is about 4 times longer (roughly estimated by me) And it wouldn't be kilometers as I estimate Protev to be 50-80 meters long (very raw estimates too from the video, no real measurements), so the Akkan would be like 400 meters long..

They are indeed communal but they like their ships to have elegance and grace and being all cramped inside is not very elegant or graceful. They connect mentally, not physically, so it's not necessary for many of them to be on the same ship because they can probably feel every other Advent in the star system.

Both of the Cobalt's and Javelis' batteries fire at the same target, each of the Garda's batteries fires at a different target. so each needs its own individual operating crew. Granted it's highly unlikely they'd need 50 to 100% more crew for just the guns though.

That's still not very definitive. Just looking at the TEC SC, judging from the cockpit windows, the bombers appear to be much larger than the fighters ( which would make sense, they're slower and carry a bigger payload ) yet in game they both have the same size. Then there are problems with the other strike craft: Advent strike craft have no windows or cockpits thanks to them being remote-controlled and some ships, like the Eradica, have no visible hangars ( or even windows ).

The resized the titan meshes multiple times during the beta. I think you're better off just dreaming up what you think the size should be rather than how they reflect/compare to each other in game.

These numbers are quite realistic, thanks for the info. We can better guess the titans now I think knowing these numbers.

This could spawn the never ending argument similar to ship lengths in Warhammer 40,000 (in which there has been no definitive statement) In fact one 40k novel had an escort at 30 kilometers....