I'm 5'10" and 145ish lbs, so I've always found 6' bags to be fine. I don't plan on any extended winter trips, just 1-2 night trips mainly here in the Tahoe basin where the night temps will generally be 0 to 10 degrees (I plan on going out when I know it won't be particulary nasty).
I plan on using a 10 degree bag with down pants and down jacket to supplement when needed, inside a tent or pyramid tarp either solo, with my dog, or with my wife and dog. I imagine I would want to keep boot liners and maybe some water bottles inside the bag, but not really anything else.
This will be my first winter camping season, so it's strange to be asking noob questions again in regard to backpacking.