Also note that by using the Tape measure tool to scale a group/component only within your model, you should first enter its editing context (i.e. double click to edit) and scale it there. In this case only the group/component will be scaled.
To scale something with the scale tool with your example (one side is known to be 6') follow these steps:
select the component then the scale tool:
select one of the grips along the axis you want to scale
press and hold Shift to scale uniformly (so that the component won't get distorted)
start the scale operation (you can now release the mouse and the Shift key), type 6' (with the unit symbol) and hit Enter.
Note that if you scale with the tape measure tool OR with the scale tool
inside the component's context, the component definition will also change while whatever you do with it from outside, the "original" definition won't change so if you insert another instance from the Component Browser, it will look like before scaling.