Note: Pretty much the same procedure can be used when rotating an object by an undefined angle.
Command: Line –> Draw a line of a length that you want to use (for example, you have an object in your drawing, and you want it to be 100 units long, so draw a 100 units long line). This is going to be your reference length. Command: Scale –> select entire drawing, except your reference line, press space.
Type “re” (as reference), press space. Pick first point and endpoint of the object from your drawing, that you want it to be 100 units long. Type “po” (as points), press space.