As I'm about to buy such a bivvy, I asked some questions to Aeromedix.
Here is the answer :
"The dimension on the Thermo-Lite Bivvy Sack when stored are approx. 6"x3.5"

The difference between the Thermo Lite Bivvy and Heatsheet Bivvy is
mainly the material. The Thermo Lite Bivvy is a much more heavy duty
item for more serious conditions. "

As for threads about bivvies, search for "bivvy" and for "bivy" !!!