I'm looking to buy a used CAAD 10 or 12 but since they are quite far away, I can't test ride them so I'll have to guess the correct size. I am 173cm tall and an inseam of barely 80cm without shoes. I did test a 52cm CAAD 12 a few years ago from my friend and since I mostly ride hardtails, I felt kinda cramped, maybe I'm just not used to. Any advice?
If you felt cramped on a 52cm then maybe a 54cm would be the better option.
You can always make minor adjustments to the stem and seat to get a better fit.
Unfortunately only you are going to know what one is better.
I’m about a cm taller than you but ride a 56cm but my bike is a Cube and their measurements are different to Cannondale so as you can see with each bike manufacturer you may need a different size frame.
I have added the Cannondale size guide for reference.