"to fit with crutches"
We are going to fit you for crutches, meaning to measure you and make sure your crutches work for you... "Vamos a tallar tus muletas a tu altura..." Is this the smoothest way to say this...?
"Medir" is "to measure" or "to take measurements". Maybe something like: Vamos a medir su estatura para buscar las muletas que le corresponden" (we're going to measure your height in order to get the crutches that fit you).
I hope this helps - I ran this by a Mexican friend and she said it would work in Mexico!