I would just use the stuff sack it came with. No compresson. Or if you want a lighter stuff sack replace it with one. With down bags over compressing them will destroy the down fibers degrading the bags warmth over time. How long that is depends on how much and long it's compressed.
With that said if you are talking a few times a year it will probably be fine as long as its stored properly when not in use. Granit Gear, Sea to Summit and kifaru all make light weight compresson sacks.
If your looking for a smaller compressed package look for bags in the 800+ fill range.
There is no Problem with compressing good quality down bags for short periods of time (i.e. hikes in and out). You just don't want to keep it compressed for weeks or months. I called western mountaineering a couple years ago about this very thing when forked out the cash for my first down bag. They told me I could compress the down bag as tight as I wanted to for short periods of time with no ill effects to the down. When you set up camp just get the bag out and give it time to loft again. The tighter you compress the bag the longer it will take to regain its loft but it will regain it every time. Actually the fibers in a synthetic bag are more likely to be damaged from compression than down is.
Yes I know not to store it in a compressed state, mostly just curious what size of a compression bag others have been using for their bags to reduce bulk for packing.I see there is a number of different sizes on the REI site.
I use either eVent compression sacks or OR dry comp sack to put my sleeping bag, extra clothes, and sleeping pad in. Keeps it dry even when the pack gets wet or submerged even. Just don't wanna risk my down bag getting wet when out on a hunt a simple slip on a creek crossing could ruin your day to say the least if your bag and clothes get soaked. I also like the OR dry comp sack as it double as a small day pack since I don't run the XTL lid on my Kifaru packs as well as being 100% waterproof. At 12 oz the OR isn't super light, but I find I like having a lightweight daypack away from comp when my bag and clothes are stowed in the tent and i need to go filter water and what not. Much comfier and holds more than the XTL lid while still being waterproof.
I really like my event compression sacs. I usually use 2. One always stays with my bag. The other holds all my extra clothing and anything else i want to throw in it. Nice thing about the event is you can continue to push air out till theres almost nothing since air will permiate the fabric. Yet it is still waterproof which is important to me.
I would really like to try the kifaru stuff sacs, i like the concept. The event sacs definetly give you the 'bowling ball effect'
Only thing that scares me w/ the kifaru is not being waterproof, i know the material is waterproof but its the seams. Although im sure it could be seam sealed just like thier tents.