Sealskinz socks are brilliant, but not for riding a bike in (unless you enjoy soggy ming feet). What fboab said.
FWIW I'm a EUR 42 and find L in Sealskinz fit well. I'm not in the habit of cramming them into cycling shoes, thobut - they're strictly a sandals and wet campsites solution for me. Barakta has a set of M, and I can fit in them, but they're a bit on the snug side and I'd anticipate a rapid hole in the membrane at the end of my big toe (this seems to be their failure mode) if I walked around in them too much.
TBH, I'm not sure the size is going to make a huge difference to how easy they're going to be to cram into your shoes, the thickness isn't going to be any different.