Totally depends upon your space. When in doubt, get the biggest light that you know you will be able to KEEP COOL in the space. This will be a trick to figure out; your exhaust is very important in choosing a lamp and the two should be purchased as a pair, along with an air-cooled hood.

Standard room sizes and lamp sizes for HPS in flower:

5x5 1000w
4x4 600w
3x3 400w
micro <400w, and in a space that small, you WON'T be able to cool your lamp properly, so stick with flouros.

You need about 6 feet tall (or more) of space for a larger lamp, not just for exhaust equipment, but also for the plants and the pots, and the requisite space between canopy and lamp.

In growroom setup, there is a sticky by Rhizome that is really informative.
stinkyattic Reviewed by stinkyattic on . yield question just started my own grow alittle bit ago i have a t5 48 45000 lumins 450 watt if i send thim into flower at about a foot and a half from veg with 5 females what would my yield come to estimations are all im expecting i saw some one on here only harvest 90 grams off of 5 two foot plants if i veg mine for three months will this greatly increase yield im hoping for about 7 ounces without using a hps i would just change my bulbs to the red spectrum but if 7 ounces is way over estimating let me Rating: 5