When I first started chastity, my cock couldn't really fit into the standard cb6000 or the standard sized holy trainer; it was too thick and I had to use nylons to pull it through. That was 2016 or so. I've been wearing regularly since then, though not 24/7 but still almost daily. At the time of start, my cock was 7.5" long when hard, although I'm a grower so maybe 3-4" when soft.
Today, I'm less than 2" soft, and hard maybe 5". Keep in mind, I wear it maybe 8 hours a day. Worth noting that my balls did not shrink at all, they are huge compared to my cock size now, to the point that it looks ridiculous. I wear a small cage now, similar to holy trainer but steel instead of plastic, and it's got a nice lock right on the ring, so not all the moving parts and pinching like the CB and HT models had.
So, yes, chastity shrinks cocks. Mileage may vary though.
I did try out flat chastity, actually it was about 1.5" inverted, so it pushed my cock inside my body. It's a really strange feeling. However, the angle for flat chastity is all wrong, which is why you see most guys in flat chastity porn wearing a belt of some type, which is to pull up on the chastity which in turn pushes your cock back into your body. If you let flat chastity hang like regular chastity, then your cock has room to move inside you and tries to get hard, which is terribly uncomfortable and feels like it could cause permanent damage, so I don't use it much at all unless I'm feeling super kinky.
I think I'm going to go for the nub next, that size looks perfect 🙂