400w will cover a 4 x 4 area which sounds more suited to your space. I have a 250 and a 400w so would tel you to go with the larger one.

As for overwatering....as long as you have adequate drainage and good soil I water mine every other day to every two or three days. You have small plants in large pots however and should water smaller amount directed toward the center (where the roots would be) more often. Water down at the very bottom of your pot will do your roots no good and would just sit there creating problems. I've found its hard to overwater FFOF unless, again, you just don't have good holes in the bottoms of your pots.