Measure an object size in a photo

[ZeitenWanderer]

Not sure, if I am right here at all ....

This is the task. I have a photo of an object (book, cd) and would like it to be masured. Now this is the setting: The camera is fixed and focussed (means some of the larger or smaller objects would not be brilliantly sharp), but the average would be just right.

Question 1 would be to cut out the actual image from a black background (could be any colour if that would help the overall task). Thought, I could handle that via the image recognition. Am I right here?
If that won`t work, there is still the more important part:

The measurement would be taken from the size of the picture (Before, I would have cut it to its actual size or in the way mentioned above). All I need to do is to deliver one object`s size in advance as a first-time setup. Next time it could tell by these settings, this size of pixel (with this set of focus) is very close to soandsomuch cm. It need not be overall exact as long as it would not fail by more than 2 mm regularly. Occasional exceptions would be ok.

Am I on the wrong track here or would it be worth following this road?