It's definitely a it more complicated than that, Spy_90. Plecos range in size from 3 inches to well over 2 feet. Most plecs kept in captivity won't exceed 15" or so, but some have!
If you're going to buy a plec, it's a much better investment to get a "dwarf pleco" such as a bristlenose or a Tiger pleco (though that's a very generic term). The best resource for info on these is
www.planetcatfish.com. Got to "E-catelogue" then Common Name or "L-numbers." "L-numbers" will take you to the most comprehensive list of all plecs on the web, and about 80% have pics and info associated with them. L-numbers, by the way, are a common way of referring to plecs that haven't necessarily been fully described, and is useful because new species pop up frequently.
Some suggestions from my end.... If you want to go with a truly beautiful pleco, consider King Tigers (L-66) Imperial King Tigers (L-333), Queen Arabesques (L-260), Gold Nuggets (L-18, 81 and 177; these do get a foot long eventually, but not usually in tanks), and the snowball varieties, such as L-142. I also happen to think that the albino bristlenoses are really cool... These don't have any 1 L-number...
These are just a few suggestions... A LOT more interesting variants exist out there... If you're willing to pay serious money ($100+) consider Zebra Plecs (L-46), and Mega Clown Peckoltias (L-340).
HTH