Someone suggested 6 x 6, so they could get 4 out of a 12 x 12. There are lots of 8 x 8 albums out there. I just found a binder and page protectors at Staples for my hard-to-find 8.5 x 5.5, which is a half sheet of paper. I love this size, but understand wanting uniformity. Post what you would prefer.
6x6 square
5x7 rectangle
8.5 x 5.5 rectangle
I like the 8.5 x 5.5 because it doesn't seem small and there is room to adequately layer and embellish. Post what you like and majority will rule. Thx, D.