Length: the longest side of the parcel
Girth: measurement around the thickest part (perpendicular to the length)
Length plus girth combined cannot exceed 108 inches. (USPS Retail Ground cannot exceed 130 inches.)

International Packages except First-Class Package International Service
Length is the longest side of the parcel.
Girth is the measurement around the thickest part of the parcel.
Length plus girth cannot exceed 108 inches.

First-Class Package International Service
Size—Other than Rolls
Maximum length = 24 inches
Maximum length + height + thickness combined = 36 inches

Size—Rolls
Minimum length = 4 inches
Minimum length + twice the diameter combined = 6-3/4 inches
Maximum length + twice the diameter combined = 42 inches

There are no defined size limits so long as articles being sent can be enclosed in the mailbag.