I have used regular index cards to write and send postcards to family while I travelled across Canada. They work great. No need to worry about the size, they are the standard size for the Post Office. Just write on one side and put the address and stamp on the other, same as usual.

On the note side you can add almost anything, but there are some rules. In this case the rules will mean your postcard is able to be delivered, or not. So take heed.

In addition, there are rules about the back side:

• The bottom .75″ must be left free of images or text.
• A 1.18″ square in the upper right corner must be free of images, to hold a stamp.
• The address of the recipient must be on the right side of the back, no more than 2.25″ from the right edge.

Source: Postcard Basics - Go Make Something