4x6...update
I like the size to go with the album, but you can't really make your pages too bulky otherwise they don't stay inside the page protector too well...I think it is just because of the size, not the product's fault.
Update:
Now that I have inserted a bunch of PL size cards along with my photos, I feel like the page protectors are a bit too small. I know they are 4x6, but the layouts seem to slide up...I think I am going to have to use packing tape (oh the horror!) on top so they won't fall out :(