My kitten's body is already almost as long as my arm. He's about the size of a typical adult moggy, I'd say, but the shelter said he's only 6 months old. How much bigger will he get?
It's hard to say without knowing what his parents looked like. Small cats tend to reach their full growth within the first 1-1.5 years. Larger cats can keep growing for another year or so after that. As big as he is now I'd guess he's going to be one of the larger cats, although that could mean long and lean rather than heavy.