ImageJ - Problem with measuring

I have a dental photos with broken files and I’m measuring this files (area, width,height etc.) but I would like to compare this values with other files from other patients. The problem is that I have 20 photos and 8 different resolutions (a few photos 934x676px, a few photos 1200x868px etc.). I know that it’s possible to change px to cm, but how I could adjust it to different resolutions to get values in this same scale. What’s the best and easiest solution ?
I don’t know any exact dimension from this photos.

Some example images would certainly help the forum members.

You should’ve at least have a scale in the image or some intrinsic file information (scanner dpi, etc.) to perform correct measurements.

If you have a kind of scale in the image (e.g., a ruler) you should be able to set the scale for each image with image, see:

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Of course I’ll show some samples but I don’t have this images with a scale in the image. For some photos I know what kind of machine was doing this photos but some photos are from google and it’s hard to find what machine was using.

Actually I find in specification that for one machine 1px = 18.5 microns

You could use the xray film as your reference as it has a known size… however I think you also need to take into account the magnification factor:
http://xrayphysics.com/radio.html
If you did not take the radiographs yourself (i.e. you know exactly the setup) you might struggle to compensate for that with confidence. You need the distance from the source to the film (maybe when using a film holder/positioner, it will be known) and from the object of interest to the film (I can’t think a way of computing that from the film alone, perhaps you could use some anatomical models as an estimate). Have you asked a professional radiologist?

Im not sure if it should be like this but I measured this object in pixels and for example it was 300px. Then I multiplied 300px x 18.5(microns) and I got in this situation 5500 microns so it’s 5.5mm. Is it good or I can’t do like this ?

Did you actually read the link? It depends on the magnification factor and at the moment we do not know what that is.

Yes, I checked it and I know what you mean but when I was doing like I told you my values were realistic because I compared this values with suspected values.