Leapard Plec*

If by leopard you mean L-114 aka Leopard Cactus Pleco, then its just under 10 inches.

If you mean
L-134 aka Leopard Frog Pleco, then its about 4.5 inches.

Not sure what other plecos might get the generic name of leaopard though many could based on the spotted patterning.
 
I've looked at all of them on the internet and its not those. This is a dark colour, almost black, with little yellow spots all over his body.
 
about the same size as a common IME
 
its not a leopard pleco then. There are several that have yellow spots, check out planet catfish and see if they have it. You know what the guy you seen looks like.
 
Do you mean this guy?
IMG_2156.jpg


If so, it is a Leopard cactus pleco(L-114) and they will grow around 13" and are a pretty aggresive and territorial pleco.
 
AquariaCentral.com