Mailing Standards of the United States Postal Service - International Mail Manual > 2 Conditions for Mailing > 260 Direct Sacks of Printed Matter to One Addressee(M–bags) > 263 Weight and Size Limits
There is no minimum weight requirement for the entry of Airmail M-bags, IPA M-bags, or ISAL M-bags. The maximum weight limit for M-bags is 66 pounds, which includes the tare weight of the sack. Lower maximum weights apply to certain countries. See the Individual Country Listings.
There are no defined size limits so long as articles being sent can be enclosed in the mailbag.