Visit the following link to view the USPS instructions for mail piece design: http://pe.usps.com/text/qsg300/Q201.htm
The following standards apply to letter size mail and postal cards:
Minimum Mail Dimensions Allowable
If you have a first class letter that fails to meet any of the following dimensions it is non-mailable.
All letters must be at least .007 of an inch thick.
At least 3 1/2 inches in height AND
At least 5 inches in length AND
Rectangular in shape.
Maximum Mail Dimension Allowable (without surcharge)
The maximum dimension standard, which applies only to first class letter mail weighing 3.5 ounces or less are:
Height may not exceed 6 1/8 inches OR
Length may not exceed 11 1/2 inches OR
Thickness may not exceed 1/4 inch OR
Aspect ratio (length divided by height must be between 1.3 to 2.5).
The UPSP surcharge of at least twenty ($ 0.20) cents, in addition to the required postage, will be assessed per piece if the mail piece is deemed "non machinable". See, also Addressing - Non-machinable criteria at:http://pe.usps.com/text/dmm300/201.htm#wp1042571
Post Card Standards
Minimum Dimensions-(if the card fails to meet any of the following minimums, it is non mailable)
Height must be at least 3 1/ 2" high.
Length must be at least 5" long.
Thickness must be at least .007 inch thick.
Height cannot exceed 4 1/4" high for the post card postage rate.
Length cannot exceed 6" long for the post card postage rate.
Card stock that exceeds the maximum dimensions is posted at the correct rate for the size, i.e. letter rate, flat rate or parcel rate.
You are encouraged to consider the above standards when planning any new type of mailing piece.