Check print size supported by your printer and add new sizes
In Microsoft Windows, go to Control Panel
Then go to "Devices & Printers"
Right click on the printer that you will use to print the envelopes, and choose "Printing Preferences"
Under "Paper/Quality", (or "Adobe PDF Settings" if printing to a file), check the list of Paper sizes, if the size is not listed, click on the "Custom" button, (or "Add" button if printing to a file), and add the size.