I calculated the beam size of the probe beam using OSCAR.

I used the distances I measured and summarized in the first drawing.

The plots show the beam waist along the optical path, cyan area is the sample, vertical lines are the optical component of the experiment, black line is a mirror, cyan line is the f=50mm lens, orange line is the small sphere f=1.25mm

The last image is a comparison of the beam spot size at the PD position with the PD size in the 3 cases.