I don't have access to your model, although I'm sure it's beautiful. Fortunately, this question can be answered without one.
A strand of DNA is a polymer consisting of monomeric nucleotide units. The nucleotides that are found in DNA are distinguished by their nucleobases: adenine (A), thymine (T), guanine (G), and cytosine (C). Each nucleotide in DNA is made up of a single nucleobase, a sugar (deoxyribose), and a phosphate group. These nucleotides form extremely long chains comprising phosphate-sugar linkages (covalent, phosphodiester bonds).
In native, double-stranded DNA, you have of these chains (or strands) arranged side-by-side such that the nucleobases on one chain are faced directly across <em>corresponding nucleobases </em>on the other chain, which enables hydrogen bonding between the bases.
There is a "rule" regarding which bases form hydrogen bonds with one another (Chargaff's rule): adenine with thymine and guanine with cytosine.
What does this have to do with the width of the DNA molecule?
The four nucleotides can be structurally classified into two categories: pyrimidines and purines. For our purposes, we only need to know that pyrimidines are six-membered rings and purines are a fusion of a six- and five-membered ring (basically, purines are pyrimidines attached to a five-membered ring). Thymine and cytosine are pyrimidines, and adenine and guanine are purines.
So, geometrically, adenine and guanine are larger (and, thus, "wider") than thymine and cytosine since adenine and guanine contain an extra ring. But remember our base-pairing rules: In native DNA, adenine (two rings) on one strand pairs up with thymine (one ring) on the other strand, and guanine (two rings) pairs up with cytosine (one ring) on the other strand.
Because there's a 1:1 ratio between A:T and G:C in double-stranded DNA, the DNA molecule consists of complementary base pairs that, in any given composition of, has equivalent widths. Put simply, the A=T pair has the same width as a G≡C pair because both base pairs are between one pyrimidine and one purine.
