First expect a bit of moisture in any waterproof bivy. I have generally found it manageable, a bit of ice or a few drops inside the bivy near the feet. I find that the few drops are generally either not on the sleeping bag or just on the outside of the shell and they brush/wipe off easily at least with a DWR sleeping bag.
The bivy might get some spray on the outside in the rain if under a tarp, but since the bivy is at very least water resistant that should be OK.
I have used a 5'x5' tarp with a waterproof bivy, the idea being to just keep rain off of the mesh window. I am now using a 5'x8' tarp. It is a bit on the skimpy side with a bug bivy, especially if you want to keep a lot of gear dry under it. Some folks like a big 8'x10' tarp.
Last edited by staehpj1; 03-20-13 at 02:50 PM.