Plecos get to be around 24 inches (61 cm) long. However, this depends on which pleco species you’re keeping. For example, the common pleco usually grows to be around 20 inches, but dwarf plecos only grow to be 2–5 inches long.
Plecos are usually sold when they’re tiny, and there’s a myth that plecos will only grow as large as the space allows them, causing people to keep plecos in tanks that are too small.
Plecos grow at a slow rate, but you should be prepared for them to grow to the average adult size of their species. For smaller plecos, a 30-gallon tank should suffice. However, if you own a common pleco or other larger species, you might need a 75-gallon tank.