Judging by the thickness of the glass, the snails on the opposite end glass and the corner seam in the background I would say that is a ten gallon tank. Needless to say this proves nothing because that pleco is not ten gallons
Ding Ding Ding! We have a winner - my first though as well. Maybe there is something on there that says it has been in there for life - but I would doubt it.
Fish will not really stop growing unless they have a "max size" and reach it - even so - most fish will grow to the point where they are awkwardly large in their inclosure and then just grow very slowly.