I love to scrap in different sizes and most especially 8x8 that are for the grandkids - with the thought not to take up too much space and storage in their parents abode.
(Laughing here) because I tend to blow out the books.
Anyway, there are times when I will use a 4x6 photo on an 8x8 page because you can get 4 - 2x3's on one 4x6 pre-cut photo paper and use as a collage, it does save photo paper because no cropping is involved unless the pictures are not in the order you want to tell the story and you still are not whacking into and using up an 8x11 piece of photo paper (some printers are picky about odd size papers-so you end up cutting it down to 4x6 and wasting quite a bit.
The photos above are 3x4 - I felt that 3x5 would've been too big and 2x3 for only 2 photos would be too small. (There are times when those sizes are the one you have to work with so just go for it). But, when you can be in control, you can chose. It particulary helps if you take and use instagram or hipstomatic photos because you can print those in 3x3 and 4x4 sizes and not lose any photo detail.
I work with Aperture as my photo program but most all of them have some kind of custom button and in windows (of which is the ONLY thing I miss from windows) there are an array of print templates that can be used for cost effective printing and collages. I love and miss collages. I do have one of the Adobe photoshops, however, its a program that baffles me and makes me crazy. And if that's what you have, there is a great tutorial and templates (that you would have thought I'd have memorized by now) on Scrapbook Etc website that are free - all you have to do is sign up on their website.