This is never the easiest question to answer with this model of CDiem (s23 - unless I'm mistaken? is the 'm' denomination reserved for the bulbous or non-bulbous toe version). Regardless, I always advise to go true to size with this model. They fit our size 48/50 model's size 45 feet perfectly, and should accommodate any true 45. I, myself, am a true 42 and wear a 42 in my own pair. However, if you are sometimes a 45, and sometimes a 46 - you're likely less of a true 45 than you think (or just a 45 with an especially high instep, or especially wide foot), and would need a bigger size in this boot. As sinnedk was kind enough to point-out, please never hesitate to contact me / call-me-out [read: ;p] directly - I love questions!