This might work…. one molecule, using the 100 nm marker provided in countless publications (yep that is a leap of faith) and independently measuring the arc distance of each of the arms of SP-D dodecamer directly from this particular micrograph (the two CRDs as rectangles with a mean of the 16 sides total, a measure of the tethered central N terminal domains, (no neck measurement because i these images they are not really discernable). The total was close, for the first sample measured and calculated per the diagram below.  Total of the parts added is just over the presumed measurement, of 100nm.