I've built about a half dozen benches over the years. Every time I moved to a new location, I'd build a new bench.
My latest bench is built with 2'x10" boards, 8 feet long; three boards deep, so I have a bench that's 30" wide, and 8 feet long.
I designed the sub structure so the bench top does not move, ever. It takes two men and a boy to lift the bench, just exactly the way I wanted it. Nothing worse than having a bench wabble around on you when weighing powder charges, etc.
I'm posting a pix of one of my loading tools, and how I mount them all on top of the bench, with large "C" clamps. I like the flexibility of moving loading presses off the bench so I can use the bench top for other work, like making knives. I have two other benches for cleaning rifles, doing stock work, and for vises I use to hold knife blades.
I like having the bench open underneath for storage, and for a place to put my legs while using a loading press.