Good all-around gear inch for winter riding?
 

I am considering building up a fixed gear mountain bike or cruiser for use as a winter beater bike, and I was curious what is a good gear inch to use in the snow? I run a 72.3 on my fixed gear road bike, which is pretty much perfect on anything.

sounds like you've answered your own question. ride the size gear you're most comfortable with and you'll have the most control in a wide range of road conditions.
I started riding in the 72 gear inch range. My legs got quicker with practice. Now I'm at 68 GI and like that even more.

I was wondering pretty much how much extra I should give myself to get through snow and stuff

For winter riding, I prefer to go very low and spin more. Last winter I was running 38x18 on my SS/FG mountain bike with 700x38 studded tires. I live in the suburbs and the snow can get heavy at times. It really depends on how much snow there is. Right now I am running 44x17, but as soon as it gets colder and snow starts falling I am gona change my front chainring to something smaller. For winter riding I like to run something around 58-61 gear inches. My experience has been that in winter time, it is better to spin lower gears then mash higher gears. Yes it means a much slower ride... but I have never been late for work.