The backprojected size of a microscope component is that characteristic size as it appears projected in the specimen plane of the microscope, once it is divided by all the magnification factors that lie between the component and the sample.
For example, the backprojected value of a pinhole with physical diameter r_phys is obtained by dividing that size by the total magnification of the detection system. This total magnification is the product of the objective magnification (that varies depending on the objective in use) times any fixed internal magnification that the microscope may contain in between:
where m_o is the magnification factor of the objective and m_sys is the fixed magnification of the system.
See, for example, how to calculate the Back Projected Pinhole Radius of a Confocal Microscope.