It is quite clear to me what your question is. If the DNA structure is a helix, then you can call that dimension its diameter.
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This value may be directly measured using an atomic force microscope to directly image DNA molecules of various lengths. In an aqueous solution, the average persistence length is 46–50 nm or 140–150 base pairs (the diameter of DNA is 2 nm), although can vary significantly.
Here is another reference
The diameter of the helix is 20 Å.
Since 1 Ångstrom = 0.1 nanometre that is consistent with your diagram.
So you can say
In a chromosome, 2 nm is the length of the diameter of the DNA.
In a chromosome, the diameter of the DNA is 2 nm.