How big does a grow box have to be to support 1 plant??

It will totally depend on the type you are growing. Are you flowering with CFL's too? If you are gonna use HPS, you will need more height than with CFL. You can train your plant also, make it grow to fit the space.
You can flower the plant early, and have a very small plant, and get away with a very small box. I have seen people growing in old computer boxes...
How much space do you have? I mean, one plant, and 3ftX3ftX5ft tall should be fine, but it all depends on the type of lighting.

I think you have maybe 6000 lumens there tops... 75w equivalent is wimpy, could be 4500 lumens. For vegging you can probably get by with a 1sf box.

If you wanted to flower with them, and them only... 10k lumens is ideal per the grow faq. So a half a square foot box.. Grow it 12/12 from seed in a 6" pot and twirl it around LST style and shove it in there.

im flowering 1 plant its on its 6 week of flowering and its about 2 feet tall but really bushy, so i think it depends on the kind of bud you're dealing with

ok thanks ima proboly make it 4 by 4 by 4 lol