Here are USAirways' rules:
>>Carry-on baggage is limited to one piece per customer, plus a personal item such as a purse, briefcase, laptop, small backpack, diaper bag or camera case. The personal item cannot exceed total overall dimensions of 36 inches, and the other item of baggage cannot exceed the following maximum dimensions:
Valet Closet--4x23 1/2x45
Overhead Stowage--10x16x24
Underseat Stowage--8x16x21
Both items cannot exceed a total combined weight of 40 pounds. Carry-on baggage allowance may be restricted due to lack of space.<<