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They don't come much larger then this. Unless you go to full rectangular bag:
Sequoia MF tall:
(w/ full collar) 6'6" 5° F 66"/61"/48" 36 oz 3 lb 7 oz
...

I have a 5 degree Western Mountaineering down bag with a Hammock Gear Winter Pheonix down UQ and have stayed warm in -20F weather.
A -20F bag might be a bit overkill, large to pack and heavy.