There is such huge variation in the size of these molecules and the size of reporting that it becomes difficult to measure where the height-peaks (LUT) and bright areas are. I have used an un-glycosylated SP-D molecule image, and 3 molecules at pH 7.4 from oft cited paper by Arroyo et al to see how much variation there is.  Vector lines, then broken and measured in parallel dimension, summed and using the bar micron marker in the publication the nm diameter and arm lengths were determined.

Mean of 12 measures (one for each arm and one for each diameter) to arrive at: n=8 arms of 4 dodecamers, plus 4 diameters: 143.8nm+/- 6.97nm. I am more than sure that adding more will change this value. But for now, anyone who wants to examine the process… here is the image.