Of course everyone has their favorite size, but I do all of mine in the 8 x 10 size. That way I can print them out on my printer and it's not that expensive even with the priemum paper. After I print them out, I put them in regular inexpensive 8 1/8 x 11 photo albums you can get at Walmart.
12 x 12 albums would be nice, but I would need to get a new printer, and the photo paper is more expensive. The smaller size seems easier to transport around and show people.