I'm confused by your question. Specifically:
1) What do you mean by 'I have a slice of field intensity at the plane of a microscope's numerical aperture'? The numerical aperture is a measure of angle, not a plane.
2) What do you mean by 'the aperture sits about 1mm from a light source'? Which aperture?
3) What do you mean by 'how do I apply the 75x magnification to both the field and the particle to reform an image with appropriate dimensions'? Why are you distinguishing between the object (the particle) and 'the field' (whatever that means...)
4) Is this a compound or simple magnifier- a compound microscope has a secondary magnifier either/both in the eyepiece and in the main optical path (e.g. Zeiss Optivar)
5) What do you mean by 'ray trace the field and the particle independently to the same image observation-point'? Again, why are you separating the object and the emitted field?