You can take something 80 inches long if it will fit into a box (diagonally) with the outer dimensions of 75in x 29.5in x 25.5in that BAAZ mentioned
How large can my bag be?
I have never understood this:-
The maximum dimensions of the bag are:
90cm x 75cm x 43cm (35.5in x 29.5in x 16 in), including the handle, pockets and wheels.
What to do with larger/bulky items
You may take bags up to the dimensions 190cm x 75cm x 65cm (75in x 29.5in x 25.5in), including the handle, pockets and wheels.
Any bags carried in addition to your free checked baggage allowance will incur an extra bag charge.
A heavy bag charge may apply to bags over 23kg (51lb).
So the maximum dimensions are not the real max dimensions, there is no extra charge for these extra big bags unless they are over 23kg or 32kg for Club, First, and OW sapphire and Emerald passengers.
Being nosy what are you trying to fly with?