If you want the smallest possible size, you're going to have to go with a down bag. A number of companies do ultralight down sleeping bags, if you can afford them:
www.featheredfriends.com
www.phdesigns.co.uk
www.westernmountaineering.com
www.rab.uk.com
Marmot make some ultralights- the Hydrogen and the Helium are worth looking into.
I've got a RAB Quantum which is about the size of a litre carton of milk and is rated down to -8C.
All of these companies offer a fair number of options (water resistant covers, fill options, etc), and PHD offers completely customised bags. PHD is probably the maker of the smallest ones- the Minimus(?) weighs a pound and is rated down to 5 celcius. That would be fairly safe temperature estimate- PHD doesn't optimistically pad it's ratings like the big outdoors manufacturers. On the downside, it doesnt have a zip or collar.
One thing to note with down is that the US and the EU use different tests for verifying down fill. This leads to the US figures being around 5-8% higher than EU figures for equivalent down. Also, the new 900 ratings used by Marmot and others are mainly due to a change in the down preparation for the US tests.
If you don't want to splash out on down, snugpack is a good and inexpensive synthetic option. Down is cheaper in the long run though as it will last far longer.