They attain their adult size by about a year; after that they'll fill out a little (but hopefully not too much!), but won't get any taller.
With my kittens, it was almost like someone waved a magic wand at age 2 and said "You are Now Grown Up." (Well, most of them at least; I have one perpetual kitten, age 3, who thinks he's Peter Pan and intends to never grow up!)
Age 2 is a generalization, of course it does not apply to *every* single cat. (Though a remarkable number of them do seem to turn into adults at 2 on the dot!) Like I said, my Spencer apparently never intends to grow up...he's a little dickens today at 3 years of age just like he was at 3 weeks!