General Conditions of Carriage - XiamenAir

What the official page says

Each Economy Class passenger is allowed to carry one piece of unchecked baggage on board; each piece shall not exceed 8 kilograms and shall not exceed 55 centimeters in length, 40 centimeters in width and 20 centimeters in height. Unchecked baggage shall fit under the seat in front of the passenger or in an enclosed overhead compartment in the cabin.