I'm working on several different albums at once in order to stave off scraper's block, but anyways, this is how I choose what size I want for each album.

FOUR BY FOUR
This is very tiny and personally I've never used it, although if you have children it could probably be beneficial, for starting off. Very cheap. I've only seen a few albums of this size for sale.

SIX BY SIX
I use the six by six if I have enough photos or memorabilia to dedicate an entire album for a single event. For example: each year of my eight day summer camp could have a six by six, but I grouped them all in one larger album. These are great because you can buy pre-packed sets.

EIGHT AND A HALF BY ELEVEN
Now we get into the "real" stuff (the books that tend to be longer and more varied). I know people say that the 8.5x11 size album is out of style, but I'm not sure that's true. 8.5x11 is great for me. First, it's a money saver. I can spend anywhere from $.50 to $2.00 on a single sheet of patterned 12x12 paper. I could easily use a paper cutter to cut down the paper to 8.5x11, and guess what? Instant scraps. I can then use the scraps for mats, borders, or whatever on other pages, and personally, I feel as though my money is going further. Also, I tend to have more memorabilia than photos, and there isn't quite as much space to have to fill on an 8.5x11, so I don't have to worry about unbalanced content. For those scrapers out there who love the craft but either don't have the time or the patience (I am progress oriented), an 8.5x11 page is quicker to fill than a new 12x12.

TWELVE BY TWELVE
12x12 seems to be by far the most common now, and it's probably my favorite if not the most practical for my personal lifestyle. The square size makes it easy to create balance, and most papers and even embellishments are made for a 12x12 page. The simple size also makes proportions and cutting easier for those like me who like things mathematically even but don't really want to have to do the math. It's especially great if you have a lot of photos or are very artistic (I'm creative, but certainly not artistic!). These might cost a little more, in some instances, but they are well worth it. They are especially great for large albums-- I use them for entire time periods or very broad topics.