Revit families allow you to create a parameter that controls a dimension, but Revit reacts differently depending on how you select and assign that parameter.
A common work flow process is to create the length parameter(s) in the Family Types dialog box, and then in the graphics window, highlight the desired dimension that is to be controlled by the parameter. After the dimension is selected, the user selects the desired parameter that is control the dimension.
If the length value has been input already in the parameter value in the Family Types dialog box. Revit will react a couple of different ways when selecting the parameter.