Hi Napper,
Use the scale tool inside the group/component (while editing it). You can also use the Tape measure tool to scale a group/component while editing it (again). Measure an edge whose desired size you know and after clicking the second time, just type the desired value.
Again, inside the group/component (this is important as beside SU remembering that it is a custom scale, your materials will also be scaled with it).
Alternatively, if it were a component, not a group (why not?) when right clicking on a scaled component, you also have a "Scale definition" menu item. That will tell SU that this should be the "normal" size and not an exceptional one. When you have windows in a model, they are likely to be in several instances anyway in which case components are preferred anyway.