how to start random maps bigger then Large?

I beleive there is a mod available that will allow you to do that.

The best games are on SMALL maps.

You can mod this pretty easily.  Is that what you're looking for?

Modding aside, I think the expansion plans include making larger maps available. Whether those will be random or not, I don't know.

Try searching the forums.

Here is the mod in question:

https://forums.elementalgame.com/436540

Make sure you enable mods in the game.