Also depends what the total fleet capacity is, as well.  Playing at a scaled fleet capacity of 1000 works wonders on lag.

Numbers of planets aren't much of an issue, its mainly the fact that when every planet has its tactical/logistics slots filled and there are trade ships all over the place with multiple race that it gets super bogged down.

Adding more and more planets (when in a late game scenario) just amplifies the problems.  So really, you can make it as large as you want but will it take forever to finish a game? YES, will it eventually become super laggy at the end of a game? YES.

yarlen had told me that there is  hard limit of 512 planet names & 512 star names in the engine.

harpo

I think so, there is no where close to 500 names defined in the string file, yet I've played (just briefly) maps of close to that size. Also if it's a custom map you can give them custom names.