After many tries, and after mainly two, but total three publications with AFM images of SP-D, some with micron markers, some with wrong micron markers, even so included, and using two methods for determining the length of hexamer arms (two trimers N terminus to N terminus) (using lines that cusp and a diameter that contacts 3 of the 4 CRD) here is an image which shows the process, a sample from each author (with variations), and the mean for quite a few measurements.

Count, N: 186
Sum, Σx: 27251.636
Mean, μ: 146.5 σ: 22.1
Variance, σ2: 491.33