How big can I go while keeping the same light?

[TheReefGeek]

Looks like you used 1/2" pvc for this PVC light stand, if you want to use the same method but bigger, go with 3/4" or 1" for added strength.

Or use wood, pine shelving is nice, and pretty cheap as far as wood goes.

The 3ft fixture I used on my 50g could also sit on the tank instead of hanging.

Take some time to think about what tank you can fit into the space you have, then once you have the dimensions of the tank we can give more specific advice. I think a shallow 3ft tank with the HQI in the middle would work, if the edges (6" on each side or so) are a bit dark for your liking, you could add some NO fluorescents for cheap just to make it look more even. You can buy the ballast, endcaps, etc cheap for NO at any lighting supply company.