Depends upon the variety and/or spp. of Geranium or Pelargonium you desire. There are literally hundreds of vars, so the tag of the particular one you fancy should be useful. And depending upon the genus/species, some don't like to be crowded.

Dan

I put one geranium in one square foot, and then plant some shorter groundcover type thing (like alyssum maybe) around it so the whole foot fills up and looks pretty!

I agree with Dan. I have several geramiums (gerania?), all in containers or the raised bed/wall that forms my atrium. The atrium one is as big as a trash can while others would fit nicely in a SFG. All depends on the variety. Also like nidiyao's idea to plant a cover of little flowers. Blue lobelia or pink alyssum would be splendid with white geranium, while white alyssum and red geranium would be great.