The size of a trolley bag is usually measured in terms of its dimensions – Height X width X depth. To measure your bag's height, place it upright and measure from the topmost part (likely the handle) to the bottom of the wheels. You can measure the width from side to side at the widest part of the trolley bag. As for the depth, you'd have to lay it flat on a surface and measure the length between the front and the back of the pack. Do note that the size of an empty bag may be different from its size when it's packed, especially in it is a soft-sided bag. Before you set out on your trip, measure your bag's size in a fully packed state to make sure that it meets the airline's baggage size requirements.