I went around and around with ZPacks about the sizing. First, the web site was misleading about length. I called that to their attention, and they immediately started changing the wording.
A medium length (5'10") does not mean that the sleeping bag is 5'10" long. It means that it is intended for use by a person of that height. So, they are making some assumptions about how much neck and head is supposed to be sticking out. I like to get most of my head down within the bag, so I ended up with a long size. Every other sleeping bag that I own is a medium/regular size and is plenty long, so this shows differences between a mummy bag and a non-mummy.
Also, don't pay much attention to the exterior length of the bag. The person goes inside, so it is the interior length that should be of concern.
After the size problems were weeded out, the bag has turned out to be pretty good for an ultralight bag.