Actually, it seems plecs can vary a LOT, not only in size but in dietary needs and waste production.
I have a very shy, predominately wood-eating, dwarf plec (in a 35 gallon) not much over 3 inches (hard to work out length when you almost never see more than the flick of a tail or a blur in retreat) and producing tiny poops, who I tend to think of as having a much smaller bioload than my female Bristlenose of about maybe 3 and a half inches (if that) who eats far more vegetable matter.
But, loosely speaking, and as One Who Knows Nothing, I'd say larger bodied - even if not necessarily enormous - plecs with bigger appetites and poops, with such increased waste production and respiration levels and a wider body shape, could probably be counted at least double (if not more) in comparison to multiple smaller, more slender-bodied fish technically representing similar numbers in inches of total length, especially regarding meat-eating plecs.
I focus on such thoughts to comfort myself for not ever being able to have big enough tanks for any of the larger plecs - doesn't work.