If the backpack does not fit underneath, it is too big. This means that the bag may measure 40 x 25 x 20 cm, 45 x 20 x 40 cm or 25 x 40 x 20 cm. If a small piece of luggage is not enough, you can take an additional one on board for an additional charge. This may weigh a maximum of 10 kg and measure 55 x 40 x 20 cm.