How big does a prostate tumor need to be before I absolutely have to get it checked out?

Check it now. If you have a prostate tumor of any size, I would recommend having it checked out. If it is a cancer, the sooner you know it the better.
Prostate tumors. Your question needs to be more specific. If your prostate is abnormal it needs to be check by a urologist.
Not big. If you have a tumor, you need to get it checked out. It is unlikely, at your age, that you have a prostate tumor, though.