A family of Microsoft word processing software products for creating web, email, and print documents.
A simpler alternative is to click the Options button in the Envelopes dialog, scroll to the bottom of the size list, and click Custom Size. That opens a smaller dialog where you can set the desired width and height.
The advantage is that you don't have to choose a custom page size in the printer driver (as long as the size is not larger than the printer's maximum). Also, if your printer feeds envelopes from a different tray than regular paper, that setup will automatically be included.
The disadvantage is that Word doesn't save custom size settings. To avoid having to enter the custom size each time, use the Add to Document button in the Envelopes dialog to make a blank envelope, and save it as a document or template file that you can reuse.