I multiple workbenches.
Main bench is a 5' x 5' table that I do most of my assembly on and also serves as an outfeed table for my tablesaw. It has a full shelf on the bottom to store all of my pneumatic and electrical tools so they are always within reach, and hooks to keep all of my clamps, straight edges, squares, glue, etc. I can get to pretty much whatever I need without taking a step.
My other benches line the walls and are 24-ish" deep to hold my grinder, sander, drill press, miter saw, etc. They have shelves and pegboard behind them for hand tools.
Things to consider:
- Plan on replacing the top as some point in time. I often find it easiest to nail/screw down scrap boards for clamping, drive a nail into the top to lay out a radius, etc. I consider it to be a "wearable" item...so I use 2 layers of 3/4" ACX plywood. When the top layer gets too rough I'll junk it and put down a fresh top. I like ACX because the top layer of veneer is very thick and durable compared to others. The big thick hardwood tops are nice, but I would feel bad for intentionally driving a screw into it, or accidentally tearing it up.
- Make sure you leave at least a 3" overhang all the way around the bench. This is invaluable when you need to clamp down your workpiece to the bench.
- Make it heavy. Even if you have to artificially add weight using concrete blocks (which are easily removed if you need to move the bench)...you don't want your bench to move...at all.
- Mock it up to decide on your preferred height. Mine is taller than most people are comfortable with, but I'm taller than most people. Make sure you consider how far toward the center of the bench you'll be able to reach into this as well.
- If you are going to use a vice, have it picked out before you plan your build.
- Seal the top very well, if nothing else so glue cleans up easier.
- Make the top as flat as your can.