Trying to force image to an actual size

[KRoberts]

Attached I have a drawing of a part that I drew from a photograph that a customer sent to me.
When I brought the photo into AD the space between the arrows is about 18" as you can see on the top ruler. However, the actual size on the part is only 5". 
My question is how do I resize the image to match this known measurement?

[Joachim_L]

4 hours ago, KRoberts said:

My question is how do I resize the image to match this known measurement?

Select the image, go to the Transform panel, lock the Aspect Ratio and type in 27,77777% into the width field and hit the Return key.

[telemax]

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[R C-R]

9 hours ago, Joachim_L said:

Select the image, go to the Transform panel, lock the Aspect Ratio and type in 27,77777% into the width field and hit the Return key.

Or let the app do the math & enter w*5/18 into the width field.  Of course, this assumes that initially the space between the arrows is exactly 18 inches, which may not be accurate enough for the OP's purposes.