Kids' bicycles sizes are indicated by the wheel sizes in inches: 12", 16", 20", 24" and 26". The size of the frame and other parts will increase according to the size of the wheels, but the right size for your young cyclist can only be determined by testing the bike in practice and assessing all aspects, the main ones being the child's skills, proportionality and safety.
The support wheels are suitable for 12" and 16" children's bikes and, when used correctly, will help your child learn to balance quickly. The support wheels should be initially secured at a level that prevents them from touching the ground when riding and only provide support when starting or finishing the ride. Gradually increase the level of attachment until the child can balance on his/her own.