When getting the message Paper size Mismatch please do the following.
1. Go to Edit.
2. Click Choose Envelope Stock.
3. Choose the envelope size desired click ok.
Before printing your envelope please make sure your printer is set up correctly.
Please use the following steps to set up the printer:
1. Click File.
2. Click Print.
3. Click the button for Envelope Feed.
4. Choose the options that best represent the way you put your envelope into your printer.
5. Click Ok.
6. Click Print.
This will resolve the printing issue you are experiencing with envelopes.