A regular-length (5' or 6') workbench is fine for most jobs. Sometimes, though, a longer one is handy, especially when you're working with sheet materials or long boards.

Build a sliding extension for your bench. Construct it as shown below from plywood and 2x4s or adapt it to match your bench design. Make the sliding panel so it will be flush with the benchtop when closed. For a heavy-duty extension, add a provision for auxiliary legs on the slider.
—Bill Selkirk, Plattsburgh, N.Y.