Size Large fits my 44.5 Sidi. Warm enough for mid-30s if you start out with warm shoes, i.e. not left in the garage. Better for dry than wet. Cleat hole lines up fine for Look/Shimano profile. I prefer the elasticized fit of these to a similarly thick P.I. shoe cover I rode before.
For wet days go with the Castelli neoprene shoe cover.