In a nutshell
The thickness of the AeroSleep pillows can be adjusted. The size you need will depend on the thickness you are looking for.
The Small pillow tends to be used for children under 2 years of age, and the Medium pillow tends to be used for toddlers aged 2 years and upwards.
In more detail
The Small pillow measures 30 x 46 cm. Depending on the number of filling units you use, it is 3, 4 or 5 cm thick.
The Medium pillow measures 35 x 50 cm. It is 4, 5 or 7 cm thick, depending on the number of filling units used.
Each pillow allows you to start with the thin filling unit. You can add the second filling unit later depending on your child's needs.