Children's bicycles are sized by wheel size 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 trying the bike out in practice and assessing all aspects, the main ones being the child's skills, proportion and safety.
Bikes with larger wheels will have a more comfortable ride but slightly less manoeuvrability, so for children with less experience and skills, it is recommended to choose a more compact model with smaller wheels.
The support wheels are suitable for 12" and 16" children's bicycles and, if used correctly, will help your child to learn to balance quickly. The support wheels should be initially fixed 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.