How big is too big?

[Burnsaber]

If you are leading the province graph by a wide margin, then you have overexpanded, since you now have that "Gank Me!" sign on your head. It is to be noted that underexpanding is just as lethal. I'd say that it is best to be the second best.

But it also depends on the situation. If many of your neighbours get into early wars/rushes with another parties, they probably won't be there to gank you -> allowing you to expand a bit more freely.

[Meglobob]

I think ideally you want to be 2nd/3rd in the provinces graph early to avoid being ganged up on.

Mid/Late game provinces don't really matter, its research/gem income that show how powerful a nation really is.