Hi and welcome! I've put together a small bit of information about cards, envelopes, and postage stamps on this page.
Let's talk envelopes first, I feel like once I explain the envelopes the understanding of the card sizes will make perfect sense. There are several different types of envelopes but we are going to cover the two most common to greeting cards. Baronial and Announcement, I bet you are already seeing the connection here. A and Bar? Please keep in mind I mentioned quick reference and I'm giving you the bare bones.
Baronial envelopes have diagonal seams, the flap comes to a point and is used for formal invitations and greetings.
Announcement envelopes usually have deep, straight flaps and side seams and are used for informal invitations, announcements, and greeting cards.
Three common sizes of Baronial Envelopes
4 Bar Envelope size is 5.125 x 3.625 - holds a 4.875 x 3.375 card, announcement, etc
5 Bar Envelope size is 5.625 x 4.125 - holds a 5.375 x 3.875 card, announcement, etc
6 Bar Envelope size is 6.5 x 4.75 - holds a 6.25 x 4.625 card, announcement, etc
Common sizes of Announcement Envelopes
A1 Envelope size is 5.125 x 3.625 - holds a 4.75 x 3.5 card, announcement, etc
A2 Envelope size is 5.75 x 4.375 - holds a 5.5 x 4.125 card, announcement, etc
A6 Envelope is 6.5 x 4.75 - holds a 6.25 x 4.5 card, announcement, etc
A7 Envelope is 7.25 x 5.25 - holds a 7 x 5 card, announcement, etc
My favorite size is an A2 Top Fold card but you'll almost always hear me reference it as a 4.25" x 5.5" top fold.
I think all cardmakers love stamps, right? I know I do. I'm going to share a few postage stamps below and give you a bit of information on the different types of stamps. Please keep in mind that postage rates change. I have no affiliation with USPS Service nor does anyone I know, to my knowledge. I am providing this information to you as a cardmaker who loves stamps. Please visit your local post office or USPS.com for more information.