In a paper about expected survival based on, among other things, average tumor length:
"Whereas the median tumor length for Gleason scores 4 through 6 was 2.9 mm, the median length for Gleason scores 7 through 10 was 13.1 mm."
I am not a doctor, but it looks to me that that statement, plus the graphs in the article, suggest 10mm limit for small.
(abstract, but you can reach the full paper from there).
Expert doctor would be the best judge.