For width, lie on your side, one shoulder on the ground, the other up. For minimum width you definitely want the edges touching the ground. I would add another six inches for "wiggle room". It will be interesting to see how often you open the zipper all the way. I had a flat, no foot box quilt which I closed off and added a foot box to make it warmer. So far I have not found it too hot in summer. My first quilt has a zipper all the way along the sides so I can close it up all the way. I never used it,so I removed the zipper. Looking at that foot zipper, however, makes me a bit jealous. I think it is a neat idea giving you a nice opening-up-option if your feet get hot.
Another nice option you have with the open foot end and zipper, is the ability to wear the quilt while standing up. I remember doing that one freezing morning with my quilt with the full length zipper. I was able to walk around camp wearing the quilt!