I have a hand that's just a little bit on the large side for a girl and I spin 3" spheres for two spheres, 2.5" spheres for more than 2 in one hand. It may feel awkward when starting, but you'll want to start learning multiples (if you aren't already spinning multiples, that is) with the biggest size you can hold. If you start with smaller ones, it will be like trying to completely relearn everything when you want to move up to larger spheres.
At least, that's what I've experienced in trying to move from 2.25" to 2.5" in a 4-ball pyramid.
Happy spinning!
~Laura

I have average sized hands, and I went with three inch acrylics. My learning curve has been a hell of a lot steeper, but I honestly think the moves I'm picking up are smoother for it.

I started with 2.5" balls, but then decided it was wrong for me. They're really small, so easier to do and learn tricks, but don't look as cool.
Ask anyone watching whether the smaller or larger balls looked cooler/more magical and I bet they'll all vote for the bigger balls. I'm using 3" now for a long time, and very happy I decided to do more work for a better effect.