Join Date: Apr 2010
Location: Near Boston, Massachusetts
Dimensions are a fairly personal decision, but I would find a bench 42" high and 36" deep to be awkward. I think my current bench is about 3' high, and it's just about right. Likewise, a 36" bench is hard to lean across, and you probably won't mostly need the depth. It looks like this won't sit against a wall, so at least you don't have to worry about reaching things hanging on the far side, but it still may be uncomfortably large when something rolls away from you.
That said, it mostly looks pretty good to me. I'd go with 3/4" ply for the shelving if you're going to do that, but I might also suggest just boxing in the spaces and not putting shelves in until you know what you need.
My only other concern is support for the top frame. It looks like you're planning on counting on screws or bolts to support the frame, and I'd be nervous about that. I'd recommend adding extra 2x4 in the corners, screwed to the existing verticals and touching both the top and bottom frames, to provide positive support for the top. I did a bench about 10' long that way, and it was incredibly solid.
For the top, I'd either go with 2x material (ideally 2x12, cut apart to provide lots of ~4-5" wide pieces with the rings running vertically top to bottom across the 1.75" dimension) or three or more layers of 3/4" plywood. My current bench uses three layers glued together, and a fourth captured by edging to keep it from sliding off that I can replace when it gets too battered. It's flat, it's solid, and it's heavy.