Measure the child accurately
Measure standing height and inseam without guessing from age. Follow the bicycle maker's measuring method because brands may define fit ranges differently. Recheck measurements when the child is near a size boundary.
Compare the exact model
Two bicycles with the same wheel diameter can have different frame dimensions and minimum saddle heights. Use the model-specific sizing chart, not a general wheel-size chart from another brand.
Check contact points and controls
The child should be able to reach the handlebar and operate the brake levers without an extreme stretch. Pedals, crank length, grip diameter, saddle position, and standover space also affect control.
Avoid buying too large for growth
Extra growing room is not useful if the child cannot start, stop, turn, or handle the bicycle confidently now. Choose within the maker's current fit range and follow its setup instructions.