I'm more of a tent person than a tarp person so I can't address your specific question. If you are going out in northern Washington state, on the PCT, in the winter, however, you might keep this partial list in mind for a tarp shelter.

(1)big enough so you don't touch the sides because they will be dripping with heavy condensation
(2) Strong enough so it can withstand several feet of new wet snow
(3) Steep enough so most of the snow slides off
(4) Tight enough so dry blowing snow can't get in

On the positive side you won't have to worry about mosquitoes.