This topic has been up several times. Do a search on pleco or largest pleco and you'll find a ton of info/pics.
I'd post a pic of my 3 footer but, the 'attach files' tool isn't working.
This is the largest one I've ever had (LD065) Panaque sp Titanic. His smile is bigger than a silver dollar. The blue is the 1/2" rear panel of the 180gal hospital tank he was in when I shot the pic. He ground up several pounds of driftwood every week: