Probably depends on the kind of Chile peppers, but if you're planting Jalapeno, go BIG!

http://www.husseh.com/hydro/wp-content/uploads/2006/05/CRW_5591.jpg

That was my Jalapeno plant 66 days after germination. It grew to be about 10x that size after a few more months, taking over that entire tray. It got so big I ended up getting rid of it, since my original intention was to grow herbs, and Jalapenos are plentiful in my area.

I would plant 16-18 inches apart for a full-bodied plant.
IÂm building a system for a friend that will be 4'x6' with the thought of planting 4 tomatoes, a couple Bells. You could always cram more, but in the long run less plants equal less work and if you can give them room they will love you for it. In this setup, I would certainly plant half light lovers around here and there; you would be surprised as to the bonus harvest.