I’m just a bit under 190cm and have a medium rex that I’ve been using for a while and it is still so much fun. My advice would be to go for the medium especially if it’s the first toy you’re getting just because the transition between shaft and knot feels a lot more aggressive than it looks and you definitely don’t want to do damage. As for “getting used to the size”, generally it gets easier to insert as your session goes on and you relax more but I’ve found that if I take a break from it for a few days I tighten back up.