Ummm I can only tell you from my experience of their growth. I've had a gibby for 4 years now and its grown for 1 and 1/2 inches to just over 8 inches if that helps.
I wouldn't give mine away for the world hes so lovely and has a personality . I've already upgraded tanks for him once and wouldn't hesitate to do it again. I suppose it all comes down to the point of do you have room for a bigger tank in the future? If so (and this is possibly because I have multiple tank syndrome) I'd buy a bigger tank for him asap.