I dont understand why someone needs to change the scale of the model in archicad...Does he-she means the working units kind of? Still dont understand...Everything is completely open in Archicad...
You need to use ArchiCAD's scale tool in the edit menu you you want to change the actual size of your building.if however you are trying to adjust the scale of a view or drawing you need to set and save the scale you want eg 1:50 in that views settings.