probably a 52-54 depending on your preference. Typically you want a CX bike to be a bit smaller than whatever size road bike you'd normally ride. Your inseam seems a bit long for your height so that might complicate things somewhat.
you don't want it to be smaller than your road bike. you want a bike that has a similar lenght TT to that of your road bike. want about the same reach, but less drop, ie your handlebars are up a bit more.
Be careful on standardized fit for CX bikes. I was about to buy a frame based on my road bike frame and for that particular frame it would have been WAY to big (weird smallish toptube but crazy high seat tube.) ve seen some frames that were as low as a 51 for me. Crazy stuff.
Be sure to check the specs before buying and make sure they are in line with what you fit already.
Im 58 with a 30 inseam.