How do I define non-standard envelope dimensions?

I’m trying to print envelopes but can’t figure out how to get it to print my envelopes properly. I’m using LibreOffice 6.3.5.2, but also get the same behavior with 5.1.6.2 on both Linux Mint 9.3 and Ubuntu 16.04.

I’m trying to print envelopes 6.5x3.63 inches. I set it up with the formatting thingie, and says it works (see pic 1).

when I go to print it, OL forces it into one of the stored envelope formats closest to the desired geometry:

Where is the envelope database, and how do I either hack the format library, or get LibreOffice to respect the dimensions I specify in the formatting gizmo?