I have a number of images in a Mathematica array, and I'd like to export the images to a PDF s.t. each image is precisely scaled to be $n \times m$ millimeters in dimension. The idea is that printing the PDF out on $8.5$ by $11$ inch ($215.9$ mm $\times$ $279.4$ mm) paper will give me images that can be measured with a ruler to be $n \times m$ millimeters.
If the dimensions of the images are not precisely $n \times m$ millimeters in dimension, is there a simple way to do this? I can also easily just apply
ImageCrop to each image in the array as a preprocessing step.