That 21 foot number you had in your head is exactly how it goes, you just keep repeating it back to yourself until it feels true. I did the same thing with my shed, swore it was 12x16 for years because thats what the guy who sold me the house said, then I went to lay down a 4x8 sheet of plywood flat and had like 3 inches to spare on the short side. Measure the actual wall to wall at the floor and again at bench height, because drywall and baseboard can eat an inch or more, and dont forget the garage door track hangs down a foot in most of these older places. If the bench is 6 feet and you only need a couple inches, you can notch the back legs or run it along a different wall, just check where your outlets and the man door swing first.

Before you even pull cable, put a scope on the panel and watch it for a few minutes during a shift change when motors kick on and off. Dirty power almost never shows up on a quiet afternoon, it hides until the building is running at full load. A twenty dollar plug in meter saved me a callback more than once.