Snot rockets, just make sure no one is behind you before you launch one. ;-)

As for cold feet, invest in winter shoes/boots if you intend to ride through the coldest months. Shoe covers are fine but there's a point where they definitely are not enough. Wools socks (I like smartwool snowboarding socks) and chemical toe warmers are also part of my cold weather foot gear.

As for sizing, if you have MTB shoes, size up with the shoe covers.