Also has to meet airline's requirements for carry on, for example, for AA:
You can bring one small carry-on bag plus one personal item per passenger as long as the carry-on bag fits comfortably in the sizer without being forced and does not exceed overall dimensions of 45 inches (length + width + height).
The maximum dimensions cannot exceed any of the following measurements: 22" long x 14" wide x 9" tall or 115cm (56 x 36 x 23 cm). All carry-on items should be stowed in an overhead bin.