Join Date: Feb 1999
Location: San Jose, California, USA
Programs: AS Plat, UA MM, AA MM, IC Plat, Marriott Gold, Hilton Gold, Hyatt Globalist
Posts: 3,161
MH has never weighed (or even measured) my carry-on bag, which is about the maximum size permitted on MH (just under 22x14x10 inches). I also carry a 16x10x9 (inch) shoulder bag with no problems.
But I mostly fly MH domestically and on short-haul international flights, so I can't say whether MH is stricter on their longer flights.