How do I find an object in image/video knowing its real physical dimension?

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0 down vote favorite I have a sample of images and would like to detect the object among others in the image/video already knowing in advance the real physical dimensions of that object. I have one of the image sample (its airplane door) and would like to find the window in the airplane door knowing its physical dimensions(let we say it has inner radius of 20cm and out radius of 23cm) and its real world position in the door (for example its minimal distance to the door frame is 15cm) .Also I can know prior my camera resolution. Any OpenCV C++code that can do that automatically with image processing? Here is my image sample