Well I heard it's 6x4.25! but if you want to make sure maybe you can consult an online printing company who produce postcards. They could be a good help for you and all your postcard issue can be answered.
If worried about the size of your postcard, a standard postcard is measured 4 by 6 inches. Postcards can actually be any size so they can be different in any country.
The standard size for a postcard is 4x6" and the biggest size is 6x11". I am not sure if there are postcards smaller than the 4x6".
No, although not illegal billing someone by postcard is not guarding their privacy and what person would want a bill on a postcard without details of the charges on it.
The postage for a postcard is valued at $0.33 as of 2014. This can vary depending on the size of the postcard and may change as the Postal Service changes its rates.
The layout of a postcard must follow specific USPS guidelines. These guidelines include size, zones that designate where the sender's address goes, and proper layout.
The standard postcard size ranges from 3.5" x 5" to 6.125" x 11.5". Any sizes that will not fit within this range will not be processed by the Post Office.
1 inch and a one tenth
To mail a first class international postcard to Canada, you will need to attach $1.10 in postage. The maximum size for a postcard is 6" in length x 4.25" in height.
The standard size postcard is 4" X 6".
As of July 2011, the rate is $0.29.
Any square unit you like can be used to measure the area of a postcard, however to make the numbers have a reasonable size and ease of handling, I would suggest using square centimetres as a postcard is about hand size (and likely to be about 15cm by 10cm).
According to the USPS website, the Maximum size for postcards is 6" long x 4-1/4". As long as you stay within this size, you're postcard will be okay to send. You may want to double check with your local post office to be sure.