I ride in sandals year round. Last week it was 22F. I wear 3 pairs of wool socks when it is that cold. I wear large but have xl to put on as outer layer -- you want room for air to create a warm chamber, which you won't get it you squish your toes with socks that are too tight.

Get waterproof socks big enough to fit over your regular wool for when it is cold and raining.