Here are some points to consider when making your fabric postcards.
1/ You are free to chose the size you wish to work with. It does not necessarily have to be the size of a postcard, it can be smaller or larger.
2/Use stiffening on the back as you do your inital sewing, hand or machine.
3/Once the design is finished trim down to required size and add a final layer of stiffening or fabric. You may consider using iron stiffening or adhere the final backing with Visoflix
4/Encase the edges with a satin stitch