Sleeping bag for OR Aurora bivy

I recently got a nice deal on an outdoor research aurora bivy. I have lain down inside the bivy a few times and I have no trouble with it being a tight fit. I'm actually quit relaxed inside a bivy.
My question is how thick of a sleeping bag can I put inside and still have loft in the bag? I think a down bag would be best and I plan on doing allot of cold weather camping. I normally wear medium wait long underwear and sometimes a mid layer fleece if needed. there doesn't appear to be much room left for a thick bag to fit inside without reducing the loft of the bag. Any suggestions?

My personal experience with bivy sacks, although limited, involved a lot of condensation on the inside of the sack - in cold temperatures (-15F) this manifested itself as a thin layer if ice. Under warmer conditions (above freezing) there seemed to be (to me) a considerable amount of accumulated water.
After discovering this (on a warmer trip) I switched from a down bag to a synthetic one when I was using my bivy sack, to keep the down from getting soaked.
Keep in mind that the environmental conditions (high humidity where I was backpacking) and my own body (I tend to sweat a bit) may have had as much to do with my experience as the bivy sack itself.

I have heard that moisture can be a problem with bivy bags but I was hopping that the Gore-Tex would remove moisture fast enough to handle the problem. I normally don’t sweet much at all so I'm not worried about that. I camp in Michigan’s Upper Peninsula primarily. So there’s generally a 100% chance of some kind of precipitation be it snow or rain. So should I consider a synthetic bag even for winter camping? I was told the best synthetic was polarguard any suggestions on this?