Measuring in at 4 x 6 inches, these discount blank flat cards are a perfect choice for save the date cards, wedding invitation inserts, post cards, birthday invitations, RSVP post cards, baby announcements and more.  The best envelope for the 4x6 flat cards is the the A4 Envelope, the second best choice is the A6 envelopewhich measures 4.75 inches by 6. 5 inches.