I got a 27.5" fork for my 26" bike, what size wheel should I run?
You probably have a different offset and a different fork height now, so whatever you do, you have already messed with the geo noticeably. 

Borrow a 27.5 and a 26 front wheel from a friend for a day, go ride, figure out which one feels better. I would guess 26 would feel closer to how it used to but it should still handle differently due to the different offset, but who knows you might prefer the 27.5 even though that should leave your geo even more altered :)