I'm sure I'll be corrected on this, but...
Common Plecos 12-18"
Common Sucker Catfish 12-24"
Gibbecepse Plecos 18-24"
The Gibbeceppse is like a Common, but with glod-ish spots on them. Common Sucker catfish have a shiney smooth back, with Common Plecs appearing to have more defined scales... Both Common Plecs and Common Sucker Catfish are often sold as Commons though, confusing stuff somewhat... Common Plecs are listed as L001, and Common Sucker Catfish are listed as L021, if you want to look the difference up on planetcatfish.com...
All the best
Rabbut