Proper Bike Fit
Measure the distance from the center of the Bottom Bracket to where the top of top tube intersects with the center of the seat tube.
This is done so that the manufacturer doesn't have to make a frame for every inch / cm, instead they only have to make a few frame sizes, and then size the bike using the components. On a bike like this, you should have 3-4 inches above the top tube.
Sizes: S, M, L
Horrible name, because if anyone should be riding these bikes it is boys, but it was dubbed during a time where dresses were more common.
Imagine where a boy's top tube would be on a girl's bike frame, that is what you use to determine the correct size.
Here is a rough guide of age to bike size. Note that kids obviously vary in size A LOT, so this is a rough guideline using wheel size with some overlap...
- Ages 3-5 12" bike
- Ages 4-6 16" bike
- Ages 6-9 20" bike
- Ages 8-14 24" bike