Adjust proper scale from dimension

Hi,

We have a drawing imported from a DXF file and its scale is very far from being
correct.

I would like to set the correct scale by setting a dimension line between two
known walls in the building and enter the scale directly if possible.

I have been playing with setting the scale from drawing options but I have
problems with the paper then disappearing completely.

What is the correct method to alter the scale of a given drawing and still have
it drawn on a normal Letter size paper?

Thanks for comments on this

regards