I am going to be building a single plant grow cab and starting some afgans(short). I am trying to figure out the dimentions of the cab, i am planning on making it 5' tall with a growing area about 3'-4' tall. what size should i make the floor plan? im thinking 2'x2', is that big enough?
It doesn't matter for your situation -- you are growing one plant so all you need to know is if the 250w will cover one plant, and the answer is absolutely. How much more area with no plants in it that the light may cover is irrelevant. 2x2 is way plenty big for one plant, in fact that footprint could hold as much as 4 plants. And you should think ahead to the future, many growers start with an idea of how many plants they will have and then after a few grows they want to expand that a bit, so better to make your grow space now to fit your future ambitions than to have to re-build at a later date. FWIW my cab is 2.5x2.5 with a 250w MH/HPS, I put 4 plants at a time in there and supplement with 8 of the 23w CFLs (2 in each corner).