My bench is 15'x24" to make use of my space. So I used 8s and 16s. 6 legs, for support in the center, and convenience to mount nailers for the top.
4x4 legs, tied with 2x4. Made it with a top and shelf underneath. Top is 3/4 5ply, sanded. Shelf is 1/2 4ply normal. The top has 2x4 "joists", 6 of them. Used joist hangers and deck screws throughout. The bottom just has 1 "joist", as I'm not pounding on it or anything crazy.
Only thing I'd do different next time is cut dados/rabbits into the posts so the screws don't bear as much load. The plywood top sits on the 4x4, but it's not ideal.
I'd also make it 3 foot deep.
When using lumber, make sure that you have ample clamps that will be able to grab your work including the bench.
When I bolted my drill press down, I used sections of 2x4 to make sure the bolts didn't tear through the plywood.
Metal is nice, but it will mess up the wood you're working on. Wood benches will get messed up by metal...
I used plywood as an inbetween. It doesn't get messed up doing mechanic things, nor does it mess up my woodworking projects.