How to change 3 meter to pixel distance ?

I have a 3D object and I need to project it relative to a 2D image, which is captured 3m away with a camera. When I tried to make a prospective projection matrix I found that I need to state the camera position from the object (3m) and the height of the Camera above the ground (1m). Thus, I need to change these values, measured in meters, to pixels so that they can be used in a projection matrix.

I need to do the computation in Matlab. Any pointers?