If you are looking to order Blank Envelopes, these can be selected during the configuration of your Greeting Card, and will automatically match the size of the card selected.
If printed envelopes are required, the sizing would be as follows:
A2: Used with 4.25"x5.5" or 5.5"x4.25" Greeting Cards.
A6: Used with 4"x6" or 6"x4" Greeting Cards.
A7: Used with 5"x7" or 7"x5" Greeting Cards.