A standard card is 4-1/4 x 5-1/2 .. you get two per sheet of cardstock
and they fit in an invitation size envelope.
Postage is .37 for a standard card... and any rectangle size card up to 6x9 ... so long as the weight isn't too much.
Square cards will cost you more... the Post Office machines cannot recognize which side is up... and cannot locate your stamp for cancelling... so they have to be cancelled by hand.
For swaps, it is easier for your hostess if all cards are standard size. Some participants don't send a large enough return envelope to accomodate larger/odd-shaped cards.
Hope this helps.