Minimum and Maximum Sizes
If a mailpiece is 1/4 inch thick or less, it MUST be rectangular and at least 3-1/2 inches high by 5 inches long.
All mailpieces MUST be at least 0.007 inches thick.
The maximum size for most mailpieces is 108 inches in combined length and girth. Standard Post pieces may measure up to 130 inches in combined length and girth (but will be charged oversized prices).
Regardless of the size of the mailpiece, the maximum mailable weight of any mailpiece is 70 pounds. Some classes of mail have lower weight limits.