I got an e-mail from ResMed today on "finding the right mask fit" that addressed my (apparently puzzling) question.
It included templates for determining proper "cushion" size. Clearly, the one I prefer is standard and the other is wide. Since switching to the standard I have minimal leaks. I assume other manufacturers have their own guides, I wonder why no sizing was done by the DME.
FWIW here's an excerpt:
Q. Where can I find mask fitting guidelines?
A. ResMed is happy to offer fitting templates for many of our masks. Click your mask below to open a PDF file of the fitting template.
Please note the following when using these templates:
Read the directions on the template carefully before cutting and using them.
The fitting template for the Ultra Mirage™ full face mask is three-dimensional and not available online. Please contact ResMed Customer Service (800-424-0737) to request a fitting template.
Mask fitting templates should be used to aid in the fitting of masks. The use of these templates is not intended to replace the advice of a medical professional, sleep technician or equipment supplier.
Altering the size or dimensions of the original PDFs in any way may compromise the quality and accuracy of these template printouts. To avoid this problem, ensure you are printing at the actual size by turning off all page scaling in your printer settings. You may also refer to the ruler within the printout to ensure the printout is the accurate size.
ResMed nasal mask fitting templates:
AirFit N10/Swift FX Nano (90kb)
Mirage Activa/Ultra Mirage II (45kb)
Mirage FX (988kb)
Mirage Micro/Mirage Activa LT (41kb)
Mirage Vista (139kb)
ResMed full face mask fitting templates:
AirFit F10/Quattro Air (60kb)
Mirage Liberty (100kb)
Mirage Quattro (170kb)