What is a 'small tumor'?
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.
https://pubmed.ncbi.nlm.nih.gov/18550474/
(abstract, but you can reach the full paper from there).
Expert doctor would be the best judge.