Sorry about being so vague.
I like ~ 128X128 for the basic enemies. (They don't have to be that size, I just like them to be within that size.)
For bosses I like 256x256. (Again, they don't have to be exact, and can be bigger than this.)
My battle system is a side-view one, so that's prolly something you should know.
If you're wondering why I didn't mention this before, it's because I added it in after making the thread. I thought the thread was dead so I never bothered to come back and edit it. ~_~