On testosterone replacement shots, and testicles noticeably shrinking. how much can they actually shrink, or disappear? reversable if off t later?

Testicular atrophy: As you take testosterone replacement, especially shots, your pituitary gland stops making the hormones that tell your testicles to work, hence the testicles shut down. The big question is whether you need to be on t replacement. Many men are placed on this for nonspecific reasons including fatigue or "anti aging". Unless you have a real reason to be on it, you should be off. See an endocrinologist.