The regular width Zpacks bag - when used as a quilt - is pretty wide. I'm a hair under 6'0 and 165 lbs and I find the width to be right on. I can sleep on my side with the quilt fully wrapped around me, so I would not want to go wider or much narrower.
As mentioned, it tapers a lot to the footbox. The footbox isn't cramped, but there's not room to spread your legs apart really. I've been happy with it. A wider footbox may be a smidge more comfortable, but it would also be more air to heat up down there. Having my feet close together keeps them warmer.
Regarding length, I got the "long" and it's a good length. Mine has a zipper to open the footbox, which supposedly uses up a couple inches Joe said. With the long I can pull it to about my eyes when I'm laying straight. If I'm sorta in the fetal position I can pull it over my head. I wouldn't want to go shorter. Longer still would be fine but unnecessary. So as long as you're okay with a fairly narrow footbox, I'd go with a regular width and long length. My preference is also to ditch the main zipper, since it's not a winter bag anyways and quilts are better IMO for 3-seasons. I added the footbox zipper so I can open it right up like a blanket in warm weather, but this doesn't work as well with the z-packs bag as it does with other quilts because the narrow footbox doesn't give you much blanket to work with at the foot end, so it's really gotta be a hot night.