Yes, common plecos should have at least a 6*2*2 ft aquarium because of their size. Plecos are territorial, so compatible fish with a full sized adult common pleco include almost all *common* aquarium fish except discus (whose tasty skin mucus plecos apparently take a liking to) and other plecos (due to aggression). In an aquarium which is big enough, common plecos can be kept with other plecos, but I would not recommend that this is attempted in anything under 10*2*2 ft in size.
Another thing to consider is that plecos are cave dwelling fish, so a single common pleco should have a choice of 2-4 caves, ideally.