- Get the "Envelopes" add-on. From within a Google Doc, click "Add-ons", then click on "Get add-ons...". Search for "Envelopes" and click its plus sign. When added, click "Add-ons" and hover over "Envelopes". You'll see a number of different sizes to choose from.
- Add some cool fonts. From within a Google Doc, click the down arrow next to the currently selected font and click on "More fonts..." at the bottom. Scroll down to discover a ton of fonts and click the ones you want added to your everyday drop down list.
I used the "Dancing Script" font for the return address, the "Great Vibes" font for the addressee and a 5x7 size on the envelope below.
A direct link to the Envelopes add-on is here: