The widest area at the bottom of the reflector is 18" across, bottom of the reflector to the top of the bracket is 19". Plus a few inches of 2x4 to brace up for the 50 lbs the thing weighs.
Here's a link to what one 1kw plasma is supposed to do in veg compared to 6 x 400 watts of metal halide:
http://www.sulphurplasma.com/biotronic.htm