That depends on the light that is used.
very true..
you can keep floros within an inch or so of your canopy with no problem... i have 4 x 44000 lumen high output floro ballasts (they are as strong if not stronger than a 400w hps, and could maybe hang with a 600w..) that i can keep less than an inch from canopy with no problem
i also have a couple 400w hps that i keep two or 3 inches above, and if i dont rotate my plants on a daily basis i get light bleaching... they are right next to the floros so i get a good comparison of how they worked compared to one another...
if you want to control height heres a few things you can do
1) as blaze said get some bushmasters or a similar product.. im in the same boat, never tried them first had.. but have read from a ton of well respected growers how well it works
2) get an indica dominant strain.. they naturally will grow shorter and bushier
3) look into some pruning techniques.. LST and super cropping are going to be the best at keeping height down...
4) think about running a scrog grow.. where you have a screen above the canopy in which you weave the branches in and out of the screen as needed until you flower...
5) and finally keeping your lights down, as was said.. just keep them as close to your plant as you can without it negatively affecting your plant.. this would be leaf tip burning, light bleaching, heat stress, ect..
those things will help keep your plants shorter and bushier rather than taller...
