I am still waiting news from Columbia but in case I am rejected which one would you choose? in terms of rank all of them are quite similar but in terms of class size, access to the faculty, living expenses, I have noticed several differences. Any advice would be greatly appreciated.

it very much depends on what you want to study... with regard to class size, GT's LLM program is gigantic, yet DC is the place where you're more likely to get a job upon graduation

I am mainly interested in International Law, according to the US news rankings georgetown is much better in this area than Berkeley and Michigan, yet I'm not quite convinced, any advice?

If your future is in academia, I suggest you pick UC Berkeley or UMich over GT, otherwise GT is the place you wanna be for searching job opportunities