You should check out michaelandpaula.com for more thinking on contact printing.

I also want to suggest that you consider 5x7. I use 4x5, 5x7, and 8x10 and I find the 5x7 the most visually pleasing because of the longer rectangle, and considerably more negatives can be contacted than 4x5, and it's no harder to carry around and set up than 4x5. A truly good compromise between 4x5 and 8x10.