= Solution
A feature of transverse size $s=0.03\,\mathrm m$ at height $H=2.0\times10^5\,\mathrm m$ subtends
$$
\Delta\theta\simeq\frac{s}{H}=1.5\times10^{-7}\,\mathrm{rad}.
$$
Choose a representative visible <wavelength> $\lambda=550\,\mathrm{nm}$. For an unobstructed circular mirror, the first <Airy disk> minimum gives $\Delta\theta_R=1.22\lambda/D$, so
$$
\boxed{D\simeq\frac{1.22\lambda H}{s}=4.47\,\mathrm m\quad\text{(about a 4–5 m mirror)}.}
$$
Using the single-slit scaling alone would give $D\sim\lambda H/s=3.67\,\mathrm m$; it gives the same metre-scale estimate but omits the circular-aperture factor. Choosing visible <wavelengths> between $400$ and $700\,\mathrm{nm}$ changes the circular-aperture estimate to roughly $3.3$–$5.7\,\mathrm m$. These are diffraction-only estimates, not a claim that resolving one stroke guarantees reading arbitrary text with limited contrast or <noise>.
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