Selecting a bike of the right size first involves measuring the user's height and inside leg length. The user can then select a bike with a frame appropriate for someone with his measurements. Most bike manufacturers provide tables that specify the appropriate frame sizes for different builds and body types.
Children's bikes are sized differently than adult bikes. Where adult bikes use the frame size as the primary measurement, children's bikes rely on the wheel size. Once a child is at least 4 feet 10 inches tall, he can graduate to the smallest size of adult bike, which has a frame size of 13 inches.