Here's the link for BPL Cocoon sizing charts --
I usually get size XL in shirts and jackets, and my XL Cocoon vest fits me comfortably worn over a mid-weight base layer and shirt without binding or being floppy loose.
As for my use of the vest, I haven't much experience with it since I usually take my Cocoon UL Hoody because it has a . . . well . . . it has a hood, which is nice for additional head warmth when used with my quilt.
If a time comes when I backpack where nights are not likely to get down close to freezing or below, I'll probably (might?) take the vest to eliminate about 4 ounces.
The insulation in the Cocoon Hoody provides outstanding warmth, and so I'd expect the vest to do the same.