Let's say I have a 8.5x11 inch full page PDF but in the center of it I have an image that I scanned that is only 3x2 inches.
What's the best way to get that imaged saved as a smaller sized PDF?
I have Adobe Acrobat X Pro.
I'm assuming because you are stating physical dimensions, you mean "physically smaller", not a smaller file size...
On your right hand draw, click Edit PDF
On the new menu bar across the top of the PDF click Crop Pages
Select your crop area and hit enter
Check your new dimensions and hit OK
If you have an small image in the middle of page with nothing else, the PDF file is only taking the image data into account in regards to file size. So, by simply cropping in on the image and resaving it doesn't result in a smaller file size. Now, if you scale the image down and then crop in on it (just to remove white space for visual purposes), you PDF file size will now be smaller. Hope this helps.