Two things that bother me about the different models is that the manufacturers are not consistent with the measurements. Sometimes it's width and others it's girth. Sometimes in the same brand. And then they get all wrapped up with height and so on. There are also two general designs. The "tent" where the roof is raised up off the body with shock cord and then there is the sock or bag with or without a headnet or cordage to keep the net off your head.
MLD Bug Bivy $125, 6.5oz, 27in at the chest, NO size options, NO custom (MLD offers a "bivy 2" which has the same size but more silnylon)
Borah Ultralight $90-100, 5.6-6.5oz, width/girth varies, customization is welcome
Bear Paw Wilderness Designs $115, 9.0oz, (price and weight vary depending on the materials), designs and width/girth varies, customization is welcome
Even though I have included 2 socks (Borah, MLD) honorable mention goes to Dutchware and Outdoor Vitals as they offer very lightweight and inexpensive hammock socks that would work as a ground bug bivy and given their size and tunnel you can raise the ends with a ridgeline. To be clear using these socks means that you'll need a groundsheet where most bivy's do not depending on the materials used. Keep in mind that the bottom layer materials can and will wear out.
UPDATE: adding another
Outdoor Research Bug Bivy $89, 16oz, One size,
Outdoor Research Molecular Bivy Sack $83, 16oz, One Size
Outdoor Research Helium Bivy $99, 24oz, One Size