Measurements on rSP-D size, two treatments in a publication by Ed Crouch et al, JBC, 1994 where they state “Electron microscopy of freeze-dried preparations demonstrated a highly homogeneous population of molecules with four identical rod like arms (46 nm in length), each with an 8-9-nm diameter globular terminal expansion”. (to add coiled coil neck – italics mine)

Their measurement of arm length seems just a little short to me since there is no bar marker in their micrographs, using their bar marker to check is not possible.  I used the width of the collagen molecule- coshadowed – to determine the magnification and two methods of measuring the arm length (diameter touching three CRD domains, and vector-node lines (obviating the need for finding the curvature of the arms)) which puts the size of their rSP-D molecules at about 135.5nm + 7nm (n=41, that i found, using excerpts from their Figures 3a and 3b).  Their assessment of 46+9=45 x2 is considerably less than 100nm thus creating a discrepancy that cannot be accounted for by adding the nm of the N termini peak to their estimates.  Overall, using the collagen shadowing width does put their image right in line with other SP-D (human and rat) molecules in terms of overall size. No difference in measurements appeared to be present between molecules treated with urea and those not.

Image below samples how I measured the rSP-D