  Precise measurements of eclipsing binary parameters and statistical studies
of young clusters have suggested that some magnetically active low-mass dwarfs
possess radii inflated by $\sim$5-15% relative to theoretical expectations. If
true, this effect should be pronounced in young open clusters, due to the rapid
rotation and strong magnetic activity of their most extreme members. We explore
this possibility by determining empirical radii for 83 members of the nearby
Pleiades open cluster, using spectral energy distribution fitting to establish
$\mathcal{F}_{bol}$ with a typical accuracy of $\approx$3\% together with color
and spectro-photometric indices to determine $T_{\rm eff}$. We find several
Pleiades members with radii inflated above radius-$T_{\rm eff}$ models from
state-of-the-art calculations, and apparent dispersions in radii for the
K-dwarfs of the cluster. Moreover, we demonstrate that this putative radius
inflation correlates strongly with rotation rate, consistent with inflation of
young stars by magnetic activity and/or starspots. We argue that this signal is
not a consequence of starspot-induced color anomalies, binarity, or depth
effects in the cluster, employing Gaia DR1 distances as a check. Finally, we
consider the lithium abundances of these stars, demonstrating a triple
correlation between rotation rate, radius inflation, and enhanced lithium
abundance. Our result---already significant to $\sim$99.99%
confidence---provides strong support for a magnetic origin of the inflated
radii and lithium dispersion observed in young, low-mass stars.
