= Solution
Relative electron–hole motion uses the <exciton reduced mass>, rather than either band mass alone:
$$
\boxed{\frac{\mu^*}{m_e}
=\frac{(0.239733)(0.720530)}{0.239733+0.720530}
=0.17988.}
$$
In a screened hydrogenic <Wannier-Mott exciton>, replace $m_e$ by $\mu^*$ and $\varepsilon_0$ by $\varepsilon_r\varepsilon_0$ in the hydrogen binding-energy scale. Therefore
$$
\boxed{R^*=\frac{\mu^*/m_e}{\varepsilon_r^2}R_{\mathrm H}
=\frac{0.17988}{8^2}(13.6057\ \mathrm{eV})
=0.03824\ \mathrm{eV}.}
$$
The effective Rydberg is an energy here, not a spectroscopic inverse length. Its scale is reduced both by dielectric screening and by the small band reduced mass. The associated radius is about $\varepsilon_r(m_e/\mu^*)a_0=23.5$ Å, several lattice spacings, consistent with using an extended-pair model as a first approximation rather than a tightly localized molecular exciton.
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