It is a good rule of thumb to choose an envelope that is 1/8 to 1/4 inch larger than your invitation card. This will allow your invitation to fit in your envelope easily without allowing too much wiggle room.
There is no most popular or best wedding envelope size. People choose many different sizes of wedding envelopes according to preference and what they want their final invitation design to look like. If we had to say which size envelope is the most popular we offer that would be our 5x7 Envelopes, more commonly known as A7 envelopes that measure 5 1/4 x 7 1/4. LCI offers an extensive variety of standard and not so standard envelope sizes to house your wedding invitations, so the sky is the limit! However, we highly recommend that whatever size envelope you choose, that you choose it first, then design your card after! This way you can be sure that there is a suitable mailing envelope available for your invitations right from the start.
The price of wedding postage varies with the shape and size of your envelope as well as the weight of invitation ensemble. A good, general rule of thumb is to avoid unusually shaped envelopes such as squares and to keep the weight of your invitation ensemble to a minimum. For more tips, view the following video: What Size Wedding Invitation Will Save on Postage?
The more you put into an invitation envelope, the more the sides bow out, and the less accommodating the envelope becomes. Wedding envelopes are made to accommodate all of the extra cards and embellishments that are often included in wedding invitations, so in most cases you can choose an envelope that is just 1/8 to 1/4 inch larger than your largest card; but do keep in mind that extra stuffed or extra thick envelopes will require special attention in sealing. For more information on bulky invitation envelopes, please view the following: Pay Attention When Sealing Bulky Invitation Envelopes!