The first and most important "rule" when working with any CAD software .....

          1:1 AT ALL TIMES  -  NEVER DRAW TO SCALE.

  While working "on screen" the actual physical size of an object is not an issue.
  

  try this ...

           insert some text into a new drawing file ... ABC123 - height 2mm - style/font etc to suit yourself.
           zoom in/out so that the text fills the available screen, save as view 1.
           now, using your text editor, change the text size to 2000mm.
           zoom out so that the text fills the available screen, save as view 2.
           difference between views 1 and 2 ? none at all, although one is 1000 times larger than the other.
    

  Scaling only becomes an issue when larger/smaller detail views are needed in the same file or when
  "hard copy" is required, in the form of print-outs.

  Detailing, new drawing setups, model/paper space and printing are all explained under their own sections.
    
  Not needing to apply a scaling factor when entering/drawing large objects and the levels of accuracy now
  possible in a drawing, are two of CAD's greatest advances (and there are not as many as you might think)
  over an experienced professional drafter on a conventional drawing board.
  It maintains a greater level of accuracy (quite apart from ease of use) on projects where many outside
  components are required for the complete model (often from separate companies around the World)
  It also simplifies the use of the many different manufacturers component libraries now available, on both
  CD and the World Wide Web, especially those from companies/countries using Imperial measurement
  standards when all you have to remember is size x 25.4 (see also, section on Drawing Units)
  A picture is worth a thousand words in any language, so we don't want to be wondering what size it is,
  it's Full Size !!

  Converting an existing "scaled" drawing back to 1:1

.

  The first thing is to get the model (the lines and circles bit) drawing back to its correct full size
  ( we will sort out dimensions/text etc later)

  Establish the scale of the drawing.

  If unknown, pick a line on the drawing for which the true dimension is known and note the figure.

  Using a properties or measure tool, establish the actual dimension of the drawn line and note the figure.

  The scaling ratio can now be established by dividing 1 by the actual dimension and then multiplying the   

  answer by the true dimension.

          i.e. (1 ¸ actual) x true.

          Example: true dimension = 400, drawing dimension = 5

          (1 ¸ 5) x 400

          = 0.2 x 400

          = 80

   So we would need to scale the drawing by a factor of 80 to return to full size (dimensions etc excepted)

   The drawing/model is now back at 1:1 (full size)

   Consideration would now be given to text height, dimension settings, required sheet size etc for the
   drawing to be finalised and completed.