I did an I Spy quilt in different sized squares that I didn't use a backing on. I don't really consider it a crazy quilt tho, I just cut the squares the way I wanted/needed so they fit together. Another idea is to use some coordinating fabrics to make bands around the smaller squares to make them equal the size of the larger squares. The bands will add interest when you sew them together.
Here is the I spy quilt I did. You can see where I have added some random strips where needed to make the squares fit together.