User can scale object in autocad drawings by reference. Scaling by reference uses an existing distance as a basis for the new size. To scale by reference, specify the current distance and then the new desired size.
user can use the Reference option to scale an entire drawing. For example, use this option when the original drawing units need to be changed. Select all objects in the drawing. Then use Reference to select two points and specify the intended distance. All the objects in the drawing are scaled accordingly.

1-Type sc in command line.
2-Hit Enter kdy
3- Select object.
4-Specify base point.
5-Type r.
6-Specify reference length.
7-Specify new length (first and second point ).

How to scale objects  in autocad