Invitation Envelopes are referred to as the "A" sizes of envelopes.. A-Envelope Sizes (inches):. A1 - 3 5/8" x 5 1/8", or 3.625" x 5.125". A2 - 4 3/8" x 5 3/4", or 4.375" x 5.75". A6 - 4 3/4" x 6 1/2", or 4.75" x 6.5". A7 - 5 1/4" x 7 1/4", or 5.25" x 7.25". A8 - 5 1/2" x 8 1/8", or 5.5" x 8.125". A9 - 5 3/4" x 8 3/4", or 5.75" x 8.75". A10 - 6" x 9 1/2", or 6" x 9.5". You can purchase Invitation Envelopes at 4OVER4.COM Maximum Size your insert, postcard or invitation should be:. A1 - 3 1/2" x 4 7/8", or 3.5" x 4.875". A2 - 4 1/4" x 5 1/2", or 4.25" x 5.5". A6 - 4 1/2" x 6 1/4", or 4.5" x 6.25". A7 - 5 1/8" x 7", or 5.125" x 7". A8 - 5 3/8" x 7 7/8", or 5.375" x 7.875". A9 - 5 1/2" x 8 1/2" , or 5.5" x 8.5". A10 - 5 3/4" x 9 1/4", or 5.75" x 9.25".