After using my bench (20" plus a hair wide, 5 ft long) for a year or so now, sometimes I wish is was longer - not too often, but that's mostly because of the scale I work - if I regularly made pieces the size of my tool cabinet, I could see wanting longer. Width, though, I've never felt like I wanted more. It seems like the few times I use more than 8 to 12 inches of depth, it's to support a more finished piece during assembly, not for really working on a piece. If I made it again, I might be tempted to make it a little narrower to save on wood, and add a tool tray to the back like the bench Roy made in his "French bench" episodes - giving me a support further out in width in the back if I needed it putting together and assembled larger piece. Next time I work on something larger (I've got a large cabinet door to replace) I'm thinking I might make some sort of frame that attaches to the the bench to help support assemblies a little wider than work well on the current bench.
One thing I'd like to build if I can make a little more room is some sort of rolling cart or folding table or rack or something I could set up closer to the bench than my tool cabinet - more often than not, my bench is less taken up with the project I'm working on and more the tools I've been using. I'm realizing that a chest on casters might have been more amenable to the way I work, but for storing the majority of my tools in one thing, the cabinet was easier to fit into the available space.
" Be willing to make mistakes in your basements, garages, apartments and palaces. I have made many. Your first attempts may be poor. They will not be futile. " - M.S. Bickford, Mouldings In Practice