Can anyone decipher what a 2m x 2m bag looks like? Ouigo Plus class in Spain allows a 3rd extra bag if it's under 2m x 2m and 30kg:

"With OUIGO Plus you can carry one piece of cabin luggage (55 cm x 35 cm x 25 cm), a handbag (36 cm x 27 cm x 15 cm) and an additional piece of luggage (*2 m x 2 m*, with a maximum weight of 30 kg)."

Anyone happen to know what 2m x 2m translates to in a 3-dimensional world?