Hi I need to buy a crate for my 2 jack russell terriers and am wondering what size to buy? Any ideas?

You will need 2 separate crates, don't keep them in the same one.
Dogs use crates as 'dens' it is their own personal space, somewhere to feel safe and secure.

A crate should be large enough for the dog to stand up and turn around in comfortably.
I personally buy ones a bit larger so the dog can stretch out while sleeping but if they are still puppies and learning to toilet outside then a smaller crate is recommended.