To make the part/feature you are editing resizable, you constrain the geometry.
You first apply geometric constraints to ensure that the geometry wont be distorted during resize. After that you apply dimensional constraints to describe the dimensions that define the size.
When you insert the part/feature in the drawing area, AutoCAD Mechanical toolset displays the Size Selection dialog box. This dialog box provides a space for you to enter values for the dimensional constraints, and the part/feature resizes accordingly.
Parts/features in a content library typically have multiple views. The same edge across different views must be associated with dimensional constraints that have the same name. Hence, do not use the default names AutoCAD Mechanical toolset uses for dimensional constraints. Instead be conscious of other the views you intend to create, and rename them in a manner that you can recall the names easily.
See the following sections in AutoCAD Help for more information on constraints.