For PB3. If you have worn the Vaporfly, can follow the US size for the equivalent EU number. Basically TTS.
If not sure, want to measure feet. Stand with your heel bone again the wall, marked the longest part of your toes from the wall, then measure the distance from the wall to the mark in mm. Then choose the size base on mm with around 10mm allowance. eg, if you measure 267mm, choose the 275(EU43) or 280(EU44). General advise is to size up to be safer for toe trauma, so EU44 is safer.
e.g I measured my foot to be 283mm. But for my Qiaodan 强风SE at 290 (EU45), I took a risk. Now my right toe keep getting hurt if I run more than 10km in it. I found I could mitigate it, by removing the insole.