Regarding the size of this crossbow, Leonardo writes that it "opens its arms 42 arm lengths" or approximately 24 metres (the stretch of the strings) and is mounted on a cart "2 arm lengths wide and 40 long". The size of the archer in the diagram gives an idea of the proportions.
The bowstring is pulled tight by a screw mechanism as shown in the bottom right of the diagram. The wheels tilt to offer a more stable base for firing.