From my own experience, you are asking a key question about quilt making....how wide should it be?
The width is determined by the width of your fabric which might be anywhere between 54" to 60". This might mean you need to add on extra material to the edges of your quilt to get the proper width. These added edges do not have to be insulated. They can just be a single piece of fabric whose function it is to stay flat on the ground as you lie under the quilt to sleep, or to be pulled or tucked underneath you.
Next, keep in mind that the widest part of the quilt will be at the top and your shoulder area, then it can taper down to your hips and legs.
So, now how do you find the width for your body? First just do some trial and error. If you have a sleeping bag, unzip it as far as you can and lay it over you on the floor as if it were a quilt. When you roll over do the edges come off the ground? Is there enough bag to your sides so that you can pull it underneath you as you lie on your stomach? When you lie on your side, shoulder up, do the edges of the bag reach the ground? If you do not have a sleeping bag try a blanket.
You are trying to figure out if there are draft spots left open around you.
Okay: Here is a "formula" as written by Ray Jardin for his quilts:
"Lie on the floor, comfortably on your side. Have someone measure your floor to floor girth at the shoulder area, which is the distance around you from floor to floor. Use a flexible measuring tape. Or use a string and measure its length. To this measurement add 12 inches for floor coverage and 2 inches for seam allowance."
Girth plus 12, plus 2 equals width.
You could be the size that means you do not have to add a width wider than the fabric. eg. 54" or 48" etc.
If in doubt make it wider you can always cut off excess later. Remember if your quilt is not wide enough you will get drafts...brrr.
Here is a nice pattern: (from the famous Neatoman)
Watch this video: