Scrapbooking albums come in all shapes and sizes. There is no one specific album size that is "best" to use, but here are some tips on the various sizes available:
The 12 x 12 papers are readily available and this particular size seems to be a good size to fit pictures and other memorabilia on the pages without too much crowding. You can also buy ready-to-make pages kits in the 12 x 12 size.
If you want a smaller size book go with a 9 x 9 or 6 x 6 size. These sizes are great for children to start with because they are smaller and easier to work with for tiny hands. The 8 1/2 x 11 size is becoming almost obsolete. There are not many scrapbook papers available in that size anymore and the industry seems to be steering away from the 8 1/2 x 11 size and going to the square sizes instead.