Accuracy for sizes

I read before ordering them left comments and I want to return to "Slippers Dotty Fish: size 12-18 months, shoe size: 20, 21 and 22" I relied on this review to place my order and I would not like others are wrong.
My child puts the slippers 20. But here are really too large. I measured therefore received the slippers to help mothers (12-18 months corresponds to a sole between 12.3 cm and 14.3 cm)
Personally I recommended the 6-12mois which I think it will go perfectly. I kept the other for later.
I can not yet judge the maintenance point of view but from the aspect of matter and slippers are great. Quality / satisfactory price