it is nearly impossible to predict the adult rat when you don't know the rat's background since the possibilities are so wide.
As Forensic said, Ratguide does have a range but you can see that that range that a male can be on the low side, half the size of the large size. Then you have some of us with rats that greatly exceed this scale (such as with my nearly 4 pound rat that is at least 16 inches long-not fully stretched & I believe another member here has one over 4 pounds, I don't recall length being mentioned)... or you hear & see of examples that fall very short of it with so many dwarf rats starting to pop up here & there.
I think it is fairly safe to say that when your rat reaches 4 to 6 months you can expect it to double in size. They are finished "growing" by 8 months but like with all adults... bulk will continue if not moderated.