As far as I am aware there is no such setting in DT; you’d probably need a piece of stand-alone OCR software. Alternatively you might try using an app like Scanner Pro to take the photos and create the PDF in future. Scanner Pro can attempt to determine the page size, and you can change the setting after the fact. To my knowledge DT will then keep that size on OCR.
For your current problem: possibly you could print the PDF to a PDF using a “fit page to” setting? From your images I’m not sure that will be possible without prior cropping.