I've got more dry bags than I know what to do with at the moment, mostly Exped in one form or another but also the Sea to Summit range are becoming favourites. They do some with an eVent panel in the bottom so you can squeeze out even more air for packing, also the compression strap models of the same are good.
Size wise I use the little 1L for clipping on my canoe thwart and keep my cigs, car key and a camera in it. I use the 5L for my hammock, carabiners and straps, the 8L for my tarp and cordage and the bigger ones for spare clothes etc. A spare top, change of trousers and spare undies for three days, wool hat, gloves and headover all go nicely into the 13L.