If you just want to plant a "SINGLE" row of tomatoes, I would say : Make is about 2ft wide. But for all-purpose planting 3ft to 4ft wide is common . With 3ft wide you can two staggered rows of tomatoes. (WWW...zig zag pattern)

With clay soil , I would recommend lots and lots of organic matter (compost, all kinds of manure, fine pine bark mulch). That should make a nice soil. I did exactly that with GA red clay soil, when I was in Atlanta.

Seysonn