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.