My bench is 3'x12'. Supported on 6 4x4 posts. 2x6 outer frame, with 2x6 "joists" spaced 12" on center. Each joist is secured to the outer frame with 2 large drywall screws, 2 20-penny nails, and a load of Liquid Nails Heavy Duty construction adhesive.
The back posts are fully 8' tall, and there's a 2x12 shelf running atop those posts.
The posts are all notched so that the load from the benchtop is supported by the wood, not by the 1/2" bolts that hold everything together.
There are "X" braces at each end to make it rigid front/back.
The top is 3/4" el-cheapo particle board with several thick coats of polyurethane. Cheap and easy to replace when it's worn out. The front edge is capped off with aluminum channel.
If I had to do it all over, this is what I'd change:
1. Wire electrical outlets into the FRONT edge of the bench. WAY more convenient than having cords running from the back wall, across the bench, to whatever you're doing.
2. I didn't interlock the joists with the frame, so they're only held in place by the 2 screws, 2 nails, liquid nails, and friction. In hindsight, I should've cut notches in the frame and corresponding notches in the joists so they lock into the frame.
3. I'd make it out of 2x8s or 2x10s instead. Just for that "extra margin".
An all-steel model would be great, except steel is very expensive and heavy. Some day, if I ever get to build the shop I've been dreaming of since I was 6 years old, maybe I'll have the capacity to put such a beast together.
-Ryan