Is there a required size that your postcard must be as per post regulations? I currently use DL 99mm x 210mm.

In Australia, my postcards have a picture at the front and room for an address and note on the back. Is that format OK to use whilst in America?
Is there a particular side that the address and/or stamp should go on?

Thanks!!

use a 4x6
addy and stamp go on same side, on the back

4x6 is best, but a US Postal Service FYI:
Postcard:
• Maximum size is 6” long X 4 1/4” high
• Minimum size is 5” long X 3 1/2” high

This is regular, not the over-sized cards.

This 4x6 is in inches. We're too lazy to convert it to metric.