Performing my weekly check of how people found this blog last week tells me that most people found it, asking the question: what size crate for border collie, in Google.
Again, in the interests of rewarding such questions, I'll do my best to answer.
We have a 42 inch crate for Ellie, and it's too big, by far. Bear in mind that dogs are usually larger than bitches, but I would safely suggest that the absolute maximum size crate you will need is a 36 inch, and you might even get away with a 30 inch for many Border Collies. It's not an exact science, as individual dogs vary within any given breed. Just don't spend extra money on a forty-two inch - it's way too big!