I have searched countless studies. None have given this number. In fact there seems to be no details of any kind about the measles virus.
I believe this paper answers your question or at least comes very close to it: https://www.ncbi.nlm.nih.gov/pubmed/6540797
I'll reproduce the abstract here, with emphasis mine to indicate the relevant portion:
The molecular length of measles virus RNA and the structural organization of measles nucleocapsids.
Full-length measles virus RNA molecules isolated from purified virions or nucleocapsids and examined by electron microscopy were 5.12(+/- 0.12) micron in length, corresponding to a molecular weight of 5.2 (+/- 0.1) X 10(6). Purified virions examined by negative staining in the electron microscope exhibited a pleomorphic range of particle sizes varying in diameter between 300 nm and 1000 nm. Purified nucleocapsids had dimensions of 21 nm (diameter) X 1254(+/- 7) nm (length) and a central core of diameter about 5 nm. Full-length nucleocapsids were composed of 204 (+/- 3) protein discs. The pitch of the nucleocapsid helix was calculated to be 6.1 nm and the helix angle, alpha, to be 8 degrees 16'. Approximate volume calculations indicate that each enveloped virus particle contains multiple nucleocapsids.
In case of unfamiliarity with the term "virion", see: https://www.britannica.com/science/virion
In case of unfamiliarity with the adjective "pleomorphic", see: https://en.wikipedia.org/wiki/Pleomorphism_(microbiology)