Merge & Print Envelopes
Available only in the Google Docs Add-on
Change the Google Document page size to an envelope size. You can do this from Add-ons->Mail Merge->Envelopes.
Select a standard Envelope Size or choose Custom to set your own Width and Height.
Click Save to apply changes to your document.
Insert your merge fields into your document template.
Select Merge to Letters and hit the blue Merge button. This will generate a document with envelopes as illustrated below
You are now ready to print your envelops as illustrated below