im not too sure on specific on plecs but i no most plecs can get to 18" unless its a bristlenose plec wich i tink gets to 7 or 8" but dont hold me to that as im not 100%. they are also fast growing and do need a large tank and do produce a lot of waste as they get bigger
we've had our plec for 18 months now, and he has grown half an inch per month. He is a big boy. Do your research, or be prepared to upgrade tanks later.
It depends what plec - there are actually a lot more small species than large ones out there in stores - BUT the small ones normally come with a bigger price tag.
If it was mottled brown and only a few £ / $ then 99 times out of 100% it'll be a common or a sail fin and it'll get to 12+ inches... in the 1 time in 100 it might be a cheap bristle nose and only get to 5-6"...