I made lots of these years ago for Christmas cards and put them in the plastic envelopes to send. They need to be hand cancelled if you choose this way. You can also stiffen them and send thru the mail without the plastic envelope.
The minimum card and letter size for US Postal Service mailings is 3-1/2" x 5". Most fabric postcards made by quilters measure 4" x 6" and can be mailed using one self-adhesive first class stamp.
Keep fabric postcards 1/8" or less thick to stay within weight limits; cards over 1/4" thick require an extra fee for their depth and will weigh more.
You can make cards that are larger and heavier, but postage will be higher. Have fun!!