The winter kit (essentially mountain pass/cold weather clothing) I put in my truck for road trips is an Extra-Large Eagle Creek, No Matter What Duffel.

I went with extra large so that I could store a full set of cold weather clothing including parka, sweaters, pants, long underwear, boots and socks -- a mix of wool and synthetic. I threw a wool blanket in there too. I don't carry this bag further than the truck; it's not so much heavy as bulky.

My other duffel bags range from large to small depending on my intention for them. Duffels are good for packing bulky items, but smaller gear can get lost in the bigger bags for smaller items should go into separate bags before going into the duffel.