Android is one poor system for printing. I'd go back to a PC for this work.
Google Docs and Envelopes add-on
Brother HL-L2340DW, monochrome laser
Brother iPrint&Scan app
Samsung Tab S SM-T813, Android 7.0, updated Feb 2018
My brother printer treats envelopes as letter size, even though when created in Docs I chose #10 Envelope.
Brother's app only allows four paper sizes: letter, legal, A4, A5. Oddly, Brother has a video showing how to load envelopes to the printer. When I went to Brother support they wanted to charge me for assistance with their printer.
Does anyone know a way to get envelopes to print correctly?