We've got a couple at the engineering/automation company I work for. They're huge, probably 8 feet long at least. No idea what they were used for - they belonged to the engineering group. Unfortunately I can't ask because they all quit.

Large-scale stuff like blueprints is the obvious explanation, but the largest plotter I've ever personally seen was owned by an oil exploration company and used to graph their seismic test results. Only about 5 feet wide (just over 1.5 m), though.