Um, depends for what, I guess.
If you want really big armies against one another, obviously Huge unit size is good. But it can drain your population if you recruit alot. And if your computer isn't that great, well... Huge unit size isn't a good idea.
On the other hand, small unit size doesn't drain your population. It doesn't let you have huge battles, though.
I always use Huge because it's more spectacular. Nothing like looking at 200+ cavalrymen charging against unsuspecting phalanxes...
It depends on your style. I don't really think there's a recommended unit size.