What is the approximate diameter of DNA as measured by Rosalind Franklin?

Rosalind Franklin measured DNA as having an approximate diameter of 2 nanometres.

That measurement describes the width of the DNA double helix, not its length. Franklin obtained structural information from X-ray diffraction patterns of DNA fibres at King's College London. The spacing and geometry visible in those patterns helped researchers estimate the molecule's dimensions and determine that DNA had a helical structure.

A nanometre is one billionth of a metre, so 2 nanometres is extraordinarily small—roughly the scale of a few dozen atoms across. The DNA helix is much longer than it is wide: one complete turn measures about 3.4 nanometres and contains approximately 10 base pairs in the familiar B-DNA form.

The diameter can vary slightly with DNA conformation and experimental conditions, but 2 nanometres is the standard approximate figure accepted for the DNA double helix. It is therefore distinct from the 0.34-nanometre spacing between successive base pairs and the 3.4-nanometre pitch of one helical turn.