you can usually stretch a halide to cover 6 sqft, more then that and your pushing it(spreading the light over too much area to keep the intensity up)
If your doing mostly softies with a few SPS you will be fine with 3 400's. If you want a good amount of SPS or want clams on the bottom, I would go with 4 of em.
And the size is great. I was going to go with that exact size, but couldnt fit an 8' tank in my new house so im going with the 72x36x27 300g.
Wide and long are good, tall not so much. I really didnt want to go over 25" tall so I could still reach everywhere in the tank without getting my chest wet...but standard size is standard size.