In reply to buzby:
You could always try a hooped bivvy bag. I use a Terra Nova single hooped one. Being on the larger side myself, I find the hoop just gives a sense of space around the head and reduces that feeling of 'confinement'.
My mate uses the Rab one. A bit heavier, but seems more robust.
neither is a cheap option tough.
The Alpkit Hunka XL offers a larger sized standard bivvi bag/sleeping bag cover, and is a cracking price. I use one of these when snow holing and find the space around the shoulders ideal for me.