  Rotation periods obtained with the Kepler satellite have been combined with
precise measurements of projected rotation velocity from the WIYN 3.5-m
telescope to determine the distribution of projected radii for several hundred
low-mass ($0.1 \leq M/M_{\odot} \leq 0.8$), fast-rotating members of the
Pleiades cluster. A maximum likelihood modelling technique, that takes account
of observational uncertainties, selection effects and censored data, and
considers the effects of differential rotation and unresolved binarity, has
been used to find that the average radius of these stars is $14 \pm 2$ per cent
larger at a given luminosity than predicted by the evolutionary models of
Dotter et al. (2008) and Baraffe et al. (2015). The same models are a
reasonable match to the interferometric radii of older, magnetically inactive
field M-dwarfs, suggesting that the over-radius may be associated with the
young, magnetically active nature of the Pleiades objects. No evidence is found
for any change in this over-radius above and below the boundary marking the
transition to full convection. Published evolutionary models that incorporate
either the effects of magnetic inhibition of convection or the blocking of flux
by dark starspots do not individually explain the radius inflation, but a
combination of the two effects might. The distribution of projected radii is
consistent with the adopted hypothesis of a random spatial orientation of spin
axes; strong alignments of the spin vectors into cones with an opening
semi-angle $<30^{\circ}$ can be ruled out. Any plausible but weaker alignment
would increase the inferred over-radius.
