true that on down being more compressible, but if it gets wet you're SOL. Sleeping bag size, and weight both depend on what kind of temps you're going to be expecting. I always get a bag that's at least 10-15 degrees below the average low in that area. For instance, say I'm going to be sleeping in an area where lows are typically around freezing, I would opt for a 20 Degree bag. To save space in your pack, get a mummy style bag because it uses less material and also increases warmth because it has less space to keep warm. A compression sack is a good idea if space is an issue, but also wears down the bag faster breaking down the loft. I use a dry sack, which is about 15 liters and it works just fine. When you're not using it, i.e. not on the road, never stuff it for storage. The more you pay for your bag, generally the lighter and warmer it will be (true to temp rating). Although, there are some economical ones out there that do a great job and only weigh a bit more than their expensive counterparts.
I love my Mountain Hardware Pinole.
Another good bag is the Kelty Cosmic, but they come in down and synthetic so choose wisely.