How do I fix size of image in digital signature?

I need the image that appears in the appearance of the digital signature to be same size as the original scanned.  I have tried saving image to pdf, png, and jpg.  When I insert signature box, it is too small.  If I make signature box bigger then the image resizes, but the text associated with the digital signature gets astronomically bigger as well and it's too big. How can I set parameters on the size that the digital signature appears so that it is the same every time I insert it instead of fluxuating depending on the size of the signature box that is drawn.  I am using Adobe Pro X.  Help!