Always draw full scale (size) in model space. Then switch to paper space (Layouts), open however many viewports you need, then scale each viewport as required. Example: a house shown in a viewport might be scaled at 1/8" or 1/4"=1'. A detail of a joist connection might be scaled at 3/4"=1'.
By doing it this way you never have to change the scale of what resides over in model space. The "scale" will always be "full size."