Bikes for children are fitted differently than those for adults. While an adult's greatest concern is the frame size of the bike, children's bikes are usually measured and fitted according to the size of the wheels accompanying them. Sizing a boy's youth bike is not much different from sizing a youth girl's bike because gender differences do not factor in much until you get into adult models. But the crossbar is often different on boy's bikes, and this can determine how easy or difficult it is to safely mount and dismount.
Measure your child's inseam. The best way to do this is in inches, because wheel size --- the common measurement used to determine youth bike sizes --- is also measured in inches. This measurement runs from the heel of the foot along the leg to the start of the crotch.
Multiply your child's inseam by 0.75. The resulting number is a good starting point when looking at potential bikes. For example, a child with a 16-inch inseam would most likely find a bike with 12-inch wheels comfortable, but at that size it may be worth your time and money to have your child try the larger 14-inch bikes because he will grow out of the smaller bike model quickly.
Choose multiple bike sizes and have your child sit on them and ride them. This is very easy in a department store, where the aisles often provide space to test-ride bikes. Your child should be able to reach the handlebars and pedal easily, and he should also be able to put his feet on the ground and dismount with ease.
Select the largest bike size that your child can comfortably ride.