How big should I make my raised veggie bed?

I have an 18 x 5ish space that I am planning to create a raised veggie bed on. The sides will be made from the remains of the concrete driveway that we will be removing. (see urbanite post in Beginner Landscaping)
I need to know whether I would be better off creating one 18 x 5 (outside dimensions - the wall will be maybe 8"thick) bed or two 8 x 5 with a path between the two. I am willing to sacrifice some growing space for the sake of convenience, but not alot, because that's all the space I will have to grow veg in.

also need to know what height i should make it. If I have enough broken concrete, I would like to have it at least 18-24" will that be comfy sitting height?

and what about the rule stating that raised veg beds should be no wider than three feet? any experiences with wider beds?
any help or personal experience will be helpful.
thanks!