MOST invitations are 5″ x 7″, which will fit a standard A7 envelope. This is the easiest envelope to find on the market either online or in store. Another invitation size, although not as common, is 8.5″ x 5.5″, which fits an A8 envelope. A8 envelopes are also called “carrier” envelopes because they are sometimes used as outer envelopes for wedding invitations (with the A7 envelope becoming the “inner” envelope).
MOST people use A1 envelopes for their response cards. The dimensions of the response card itself should be 4.875″ x 3.375″. One nice thing about this size card is it can be printed four per 8.5″ x 11″ page (saves on printing costs). An alternate size for response card envelopes is A2, with the dimensions of the card itself measuring 5.5″ x 4.25″.