From my experience working on projects, deciding on detailed specifications may seem to be the most daunting task, but becomes clear-cut the more you are familiar with the process. The most important factor when specifying ranges of specs is the limiting conditions involved in the design input. I work on designs for a Solar Car team, and when we decide on design specifications, the ranges for design inputs are either based on earlier inputs that we set as a condition to meet or design regulations. For example, FSGP regulations requires that the vehicle must have two inches of ground clearance, so this sets the lower range for many design specs that will go into the suspension systems of the solar car. Sometimes it is based on the objective of what we want to achieve with a next generation solar car. Say, we want straight-line efficiency to take precedence over cornering efficiency in the next generation solar car, then the tolerance for specs that directly affect the rolling resistance (leaning of a car while cornering) can have a wider range without significant concern. Another factor that helps decide on a range is simply following standard equations that are used in specific designs that can help allude to desirable ranges of a design input. There are several design limitations that are already in place that helps deciding on detail specs a lot easier.