Hello Elisabeth If the distance you are talking about is between the object falling and the surface it will impact, you don't use distance to simulate falling from that distance. Instead, you put the two objects in contact and assign an initial velocity to the falling object to simulate the drop height. For example, the impact velocity in m/s for a drop height h is sqrt(2gh) where g is 9.8 m/s^2 and h is in meters. It should be simple to make that velocity a parameter in Mechanical.
If you want to know how to set up a parameter for a dimension in SpaceClaim, check out the video in this discussion: