= Solution
The <distance modulus> at $100\,\mathrm{pc}$ is five magnitudes. Thus the two approximate main-sequence endpoints read from the original figure, $m_V\simeq0,20$, correspond to $M_V\simeq-5,15$. With the given solar reference,
$$
\frac{L_V}{L_{V,\odot}}=10^{-0.4(M_V-5)}\simeq10^4,10^{-4}.
$$
Use the figure's approximate endpoint <effective temperatures> $25{,}000$ K and $2500$ K. If $V$-band <luminosity> is used as a rough bolometric proxy, the <Stefan–Boltzmann law> gives
$$
\frac{R_*}{R_\odot}\simeq
\left(\frac L{L_\odot}\right)^{1/2}\left(\frac{T_\odot}{T_{\rm eff}}\right)^2.
$$
Taking $T_\odot\simeq5800$ K gives
$$
\boxed{R_{\rm small}\simeq0.054R_\odot,\qquad
R_{\rm large}\simeq5.4R_\odot.}
$$
These are schematic estimates, not precision <radii>. The diagram supplies $V$ magnitudes rather than bolometric <luminosities>, and hot and cool endpoints need different <bolometric corrections>. The exact range cannot be recovered without those corrections; the quoted values state the intended approximation explicitly.
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