I just started putting my little guy in one - he's just over 4 months old, 16+ lbs, 25" in.
At home, I have a Rainforest Jumperoo - at daycare, they have the exersaucers. He fits great in the jumperoo, but he needs a little blanket help in the front in the exersaucer. He only spends about 10 minutes in either right now simply because he hasn't quite figured it out yet. I can say that he seems to prefer the jumperoo though and I feel like it's a better fit - you may want to consider that before purchasing.
I think it really depends on how big your little girl is and whether or not she can hold herself up in it. He holds himself upright quite a bit and really enjoys the jumperoo when he's in it.
On another note - isn't it so fun!!?? :) This is my first child, so I'm just soaking it all up!