All manufacturers measure the size of the suitcase from the ground to the top of the carry handle. So this means they don't actually give you the internal measurements of the suitcase. It is easiest to measure your suitcase when it is standing upright. This will allow you to measure its exact height with wheels, from the bottom of the floor to the top of the handles. You need to take those into consideration too, as well as the measurements of anything protrudes on the sides – zippers, handles, TSA locks etc. because all of those are important when calculating the linear measurements of your luggage. This article on how to measure trolley bag size will help you out.