I use 12 x 12. I usually have about 4 photo's per page, but do print them off smaller if I need to. I do all my own printing, as I find the photo labs do a bit of a poor job with cropping digital photos, plus they are so expensive.
Disney's Wonderful World of Memories in DTD is great. The kits are a bit pricey if you are buying a few, but I think they are excellent. The books I have had all are post ones, they unscrew and you can add pages. What I did this year was buy it all in this one spot and have it shipped home. Because I didn't have to pay sales tax, as it was being shipped out of state, I only really ended up paying an extra $5 to have it shipped home, rather than taking it with me in my suitcase. It arrived about a week after I got back. I bought a lot, so it was pretty heavy, just the scrapbook on it's own was a bit hefty. You can buy the scrapbooks and kits at every park somewhere (The Emporium at MK for example) and also at the resorts. There are some great stickers and punches there too. I personally love those Jolee destination stickers (Michaels have them) they are great for Epcot World Showcase, though I need to persuade them to do a Norway one. The DTD store actually had some in stock. Jolee's also have some great Disney stickers too, paper and fabric ones, that have a bit of dimension to them, they look great. And don't forget that at the DTD scrapbook store you can actually send a postcard with a special cancellation mark. I'm not sure exactly what it says, but it's the only place you can do this.