= Solution
The center-to-center extent is $18\,\mathrm m$ on each side and $18\sqrt2=25.46\,\mathrm m$ diagonally, while the shortest nonzero baseline is $6\,\mathrm m$. At $\lambda=0.02\,\mathrm m$, characteristic scales are
$$
\boxed{\theta_{\rm syn}\sim\frac\lambda{b_{\max}}=2.70',\qquad
\theta_{\rm large}\sim\frac\lambda{b_{\min}}=11.46',\qquad
\theta_{\rm primary}\sim\frac\lambda D=17.19'.}
$$
These describe the finest synthesized scale, a largest recoverable scale estimate, and the individual-dish field of view, respectively. Along an array side the longest baseline is only $18\,\mathrm m$, giving $3.82'$ rather than the diagonal estimate. Projection, weighting and sidelobes determine an actual <synthesized beam>.
There is no sharp universal largest-scale cutoff: the commonly used $0.6\lambda/b_{\min}$ convention would give $6.88'$. For an ideal uniformly illuminated circular dish, the primary-beam full width at half maximum is approximately $1.02\lambda/D=17.5'$, while the first-null angular radius is $1.22\lambda/D=21.0'$. These definitions should not be mixed. Uniform lattice spacing also produces grating sidelobes; the <primary beam> restricts the useful field but does not supply the missing zero-baseline <flux>.
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