I am also new to this fields but maybe I can be of some helps. I don't know if you resolve your problem since it has been a few days.

I am not sure to understand what you mean by

I want to normalise the bounding boxes so that objects of the same size appear similar in dimensions, regardless of their distance from the camera.

you want the object to be at a fix size? or the bounding box is at a fix size regardless of the size the object appear to be (the position of the object to the camera)?

If you just want to modify the size of the bounding box. You could do a depth estimation https://paperswithcode.com/task/depth-estimation and do a cross multiplication.

If you want to modify the size of the object, you could try to extract the object of the picture, resize it and reintegrate it. maybe there is easier way.

hoping I could help.