58 inches wide and 84 inches long .... oversized so I don't have to mess with straps, or tie downs, or whatever. I stuff the quilt in the Bivy, slip into the quilt at bedtime, and no matter how much I toss and turn during the night (which is a lot) the Quilt doesn't move off of me. The quilt weighs in at 18 oz .... I calculate that I could reduce the size to about 54 x 76 and get it right at 16 oz for this design.
Smaller size quilts were always getting twisted around for me, or I'd get a breeze up the backside sometime during the night.
No Quilting ... I stuff the Bivy and Quilt together into the bottom of my pack ... and pull on the bivy to take it out of the pack ... saving the insulation from getting pulled. No quilting means that other than just being careful with it, you also can not machine wash the quilt ... only hand wash and hang dry for this puppy, but a small price to pay for an unbroken momentum top.
Lastly ... a zipperless bag, like the design by Bill F. would be a more efficent design if you don't like the varible insluation cross section and would eliminate the need for a Bivy. YMMV