When you go cruising for softboxes, you'll note that there are lots of sizes -- but also several shapes.
If you're just doing portrait work, a medium, rectangular softbox might do. If you're lighting up a bit more -- then (as Glen points out) bigger is better. However, if you're lighting a person -- maybe for a glamour or fashion shoot? -- then you should look at the strip domes.
When I had a studio with a partner, I often used my medium softbox for portraits and used his large softbox for full length. I didn't like the light spill, so I'd often tape foamcore panels to the face of the softbox to create a strip. When we split, I bought a stripdome.
Think about how you're going to use that light. If you're doing more than faces, get something that lights more than faces.
Besides, if you're shooting faces and are moderately handy you should build one of those Keno-style lights that Rolando has been promoting.
Teresa using that Keno-style light.