What is the best size grow box?

A grand or two? You can buy a 4x4x7' grow tent for ~ 200 bucks.

600w covers about a 3x3 area. I recommend a 1000w air cooled hood with a 5x5 box/tent

 

The more space the better though. I'd make it as large as possible or use the room itself. You'll need a 400+ cfm 6-8" fan, but you can always move the light and plants to the center if you have extra room. Then, if you want more plants, you can add another 1000w to cover a larger area

 

I'm going to disagree slightly. Yes, a 600w covers a 3x3 area wonderfully. But is it the most efficient area to cover? I don't think so.

Too many growers hurt their GPW (gram per watt) efficiency by putting too much light in their area.

I did an experiment a while back in a 4x4 tent with a 600w and 1000w. Using clones from the same mother, the 1000w did yield more than the 600w, but not significantly so - I can't recall the exact numbers, but it was around an oz more with the 1000w. So you're paying for 400w of electricity, and only getting an extra oz when you can run a separate 400w bulb and get a half lb under the light.

I've never run a 1000w in a 4x4 again since then, and my GPW rating shot through the roof.

My personal ideals, using lighting efficiency as a standard are something like this:

400w - 3x3
600w - 4x4
1000w - 5x5 or 6x6 (I prefer 6x6, personally, I just get more out of my light that way)

Tom Hill, the breeder, runs a single 1000w in his 8x8 tent and yields very well. That's stretching it a bit for me.

 

I totally agree with highonthehill

250w - 2'x2'
400w - 3'x3'
600w - 4'x4'
1000w - 6'x6'

you can put some extra plants around the border if you have room though

 

I built my first tent out of 2x4's, plywood, panda film and Velcro. After all was said and done I wish I'd have spent the $200 on a tent. I have one now and it's much easier to work with, take down, move and modify my setup. I'd buy a 5x5 and run your 600 in there with a big ass square vented reflector. Mount a 6" vortex fan directly to a nice scrubber, Zip tie it to the top of the tent and exhaust through your light hood and out. Use some 8" ducting to create a light proof passive intake at the bottom of the tent and your golden.

If your space will allow it this is absolutely the way to go IMHO.

 

I use a 4x4, started with a 400w HPS and then went up to a 600w.... Tents set up easily, and if you need to move or take down for a while they are easier than homemade ones. Also they already have multiple vent areas that can be used or just velcro cover for them. They have reflective material built into them, and easy to hang lights, ect from top. Depends on your plans for the future, but if you plan on moving (or moving grow area) I would go with store bought if you are not sure what future plans are.
I have added 2 - 240w led panels for current grow with the 600w HPS, but previously used 2 large CFL's for side light. IMO, the 600w doesn't cover 4x4 effectively by itself, but for cost, lumen output, heat generation, height from top of canopy that you can run, ect.. it is a better option than a 1k light by itself in a tent setup. The use of movable side lighting with a 600w hps has worked well for me.

 

Hmm i was considering but this is gonna be my first ever grow i think 4 feet should be plenty tall flowering plants at 12" when finished should be around 2-2.5 feet plus 10" for pots and 5 inches for light = 45" also going to be doing uncle ben topping for 4 main colas should yeild atleast an oz a plant but just wanted to see peoples thoughts. side question, have always been wondering if it is worth the money to invest in any of these grow boxes