How about something like this? These are 10' x 4' x 8" raised beds at the Briggs Avenue Community Garden in Durham. I like this design because it has a 4" x 4" corner piece on the inside that helps hold the bed shape. Nail it all together and put cardboard on the bottom to inhibit grass growing through. Viola! They are good size for a family too.
Also, I just saw a presentation from this organization (Growing Hope) at the American Community Gardening Conference in Atlanta and they build 4' x 4' raised beds with a trellice. Very simple design and nice and small and compact. http://www.growinghope.net/programs/gardening/raisedbeds