To help you in your choice of size, a chart is available for each collection of slippers (baby/child/adult). 

The measurement of the soles indicated for each size is in cm, and corresponds to the EXTERIOR measurement and not the interior of the slipper. 

I advise you as much as possible to measure the feet to be put on. Then, if you want a slipper adjusted to the current size, you can go with a slipper sole 1.5 to 2 cm longer than the measurement of the foot (2 cm if your criterion is that they must be long). 

🦶 Simulation: Mathis wears 8.5-9US in shoes (be careful! Shoe brands are not all made the same and don't be fooled by winter boots!), his foot measures 15.7cm when drawn on a sheet of paper and then measured from heel to toe. So I opted for size 4 (9US) which have 17 cm of outsoles. They are perfect, still room for the toes to grow but not too big either! 

I am always at your disposal for questions via my Facebook page :) It is also possible to make an "in-between" for the sizes!