I would upgrade your tank right now if you can, the plec will grow aorund an inch+ a month until it gets to around 12-15inches as Kathy said (their growth can slow down before then or even keep steadily growing after 15inches- with sailfin and common plecs, it can be kinda hard to predict as each plec is an individual- my female sailfin was opposite, and hardly did any growing when i used to have her in my old 50gal, but since she's been in my 125gal for the last 8months her growth has really started to accelerate again, i expect her to grow quite large).
So yeah, my advice to you is to upgrade as soon as you can- get the most gallons you can posibly afford/fit, as a 5ft by 2ft by 2ft tank (or just under) is really the minimum size for sailfin plecs.