It is a 15". I have tried searching the threads, but only found 1 obvious thread. I tried looking up best cases, but I don't know how to do a phrase search, so it gave me every thread with either word in it and not both.
I think a majority of people will recommend either Speck, InCase, or Invisible Shield. I'm certain there are at least a few more out there (Twelve South's BookBook comes to mind) but the first 2 are the popular ones.