I started doing the-year-in-pictures collages a few years ago, and they were such a hit with friends and family that I've since gone back and done them for earlier years, and plan to do more. There's just something about being able to see the significant events for a full year in one place that makes them irresistible to look at.
I've varied the number (and hence average size) of separate images in them from 40 to 70, and settled on 50-60 as best. Fewer than this is not as interesting, more than this, they get too hard to see. Done best, they have a treasure hunt sensibility to them. I've basically wallpapered our rear hallway with them, where you can see the kids growing up and the parents growing old. Virtually every first-time visitor expresses an interest in doing something like this with their family pictures.
Now an official family tradition, the kids start nagging me for the new one every January. My son just told me that my latest effort (2006) doesn't have enough pictures in it, and I think he's right. I used to print them at the largest size my printer could handle (13x19"), but I've since switched to 16x20" from an online printer, and this has probably upped the optimal number of images. These used to be very time-consuming to produce, but Easel's new auto-arrange cabability makes them a snap, so I'll probably listen to him and redo it.