I have made a few, and screwed up a few too.
I finally got over trying to make mine 12 oz and decided I wanted good coverage.
With a footbox, make it as tall as you are or maybe even add a couple.
If a drawstring footbox then you should add length for that.
Make sure the footbox is big enough. I wear a size 12 and made mine about 13" in diameter. Socks booties etc, and I did not want to press out the insulation and I wanted not to be constricted. That said I think most are around 12" in diameter.
If you are a side sleeper then you need a wider quilt since your knees and butt will be hanging out the side. If you sleep like you are in a coffin then you can make it more narrow IMO.
For me I find that I like a hex shape. IE narrow top. straight sides then taper at the feet.
The golite ultra 20 is more of a coffin shape.
The widest width at your torso should be as wide as your girth around your arms. Seems like I read Tim Marshalls recommendation of that but not sure.
At any rate I am 6'-3, about 230 today.
Both of these work for me. One is the Golite ultra 20 long, that I measured when I owned it. The other one is my latest XP/M50 quilt size. That is the one I am sticking with. I like the extra width but it adds a bit of weight.
The latest weighs 14.5 oz with 2.5 apex and M50 with a zipper and one side of the apex inside covered with 5mm silk.
It is a modular quilt.