I have found sizing to be similar to adults with a few caveats. When they are first starting out, ensure that the seat can go low enough that they can comfortably reach the ground while sitting on the saddle. This removes the fear of not being able to get themselves balanced when sitting. Whether you go the push/run bike or training wheel route, being able to comfortably get their feet on the ground makes for a lot fewer crashes. I think you will be pretty amazed at how effective the low saddle/no pedal push bikes are at teaching riding. It removes the steering/pedaling/balancing coordination problems and lets them concentrate on steering/balancing. Raise the saddle as they get better and more comfortable with riding until they are at the preferred slight bend in the knee height. How long it takes to get to that height will depend on the child. Could be as short as a few weeks or as long as a year. My experience is that the earlier they learn, the longer they need to spend with a lower saddle. But that is a rough generalization. My measure, is that if they are falling over too frequently, they need the saddle lower until their skills improve.
At 45", she is probably tall enough for a 20" maybe even a 24". However, she is still not riding, so there is probably a disadvantage to moving her to a bigger bike. The bigger bike will be harder to control and more intimidating. It could slow her progress. Without seeing her or how she rides, I'd say keep her on the 16" until she can ride comfortably, then get her a bike that better fits her. Kind of inline with your thinking. Although I would even caution against a November bike purchase. She may have a growth spurt and be ready for 24" by the following Spring (assuming you don't get much riding in during the winter).
I prefer gears also, but she likely will not be very good with them. You will likely need to direct her on which gears to use and when. Typically until they are about 8 or 9.
I would also recommend a bike without suspension. They are pretty useless on kids bikes. There are a few geared non-suspension bikes around but not many. You may have to search for one.
Last edited by masiman; 05-22-09 at 02:16 PM.