Firstly, never import a dxf file into an existing drawing...
Then, when you import a dxf file and you can’t get the scale right, find an object that you know the size of, something with a dimension on it.
Measure that object after importing.
From the Menu Bar choose Modify > Scale Objects...
Rescale the drawing to suit using the Scale Objects command.
Select the options:
Enter the scale ratio as A/B where:
A= the original size of the object
B= the size of the object in the imported file
Click on the OK button and the draw will rescale everything.