According to AMuS, the C42 is by far the shortest car in the field:
"What you don't see at first glance is the length of the car. According to information from auto motor und sport, the Alfa Romeo C42 is by far the shortest car in the field. The wheelbase, which was limited by the regulations to 3.6 metres for the first time this year, is said to have been cut by the Sauber engineers by another 9 centimetres.
The trick helps the aerodynamics and saves weight. The Alfa is said to be one of the few cars in the field that made it exactly to the limit of the maximum weight of 795 kilograms. During testing, some of the competitors were still lugging around excess weight in the double-digit kilogram range."