Envelopes will format your document to a variety of common envelope sizes or you can create a custom size.
Envelopes helps you work from the cloud by adding the ability to print envelopes from a Google Document. This handy add-on sizes your document to the dimensions of popular envelopes and formats the tabs so you can quickly fill in Addresses.
After setting the printer dialog to the correct envelope size you will wonder how you ever got along without this feature.
There is also a custom option that allows you to set your Google Doc to any size required allowing you capabilities far beyond the envelope.
If your printer doesn't support the selected envelope size you can select the Landscape option in the printer dialog to get great results.