The best advice I can give you is to go to a bike store (even if you're not planning on purchasing a bike there), and have them fit her.
Our son started with a 12" at 2.5 years. He's completely outgrown it. We're planning on getting him a 16" for his birthday in a few weeks. We've gone to a few bike stores, and that has been the advice from them to accommodate growth, removing training wheels, etc.
One of our neighbors got her son an 18" because it was on sale. He's 4.5, and it's way too big for him. He won't be able to remove the training wheels for a few years at this rate because of how gigantic it is compared to his body.