Rotation periods from Kepler K2 are combined with projected rotation velocities from the WIYN 3.5m telescope to determine projected radii for fast-rotating, low-mass (0.1<=M/M_{sun}0.25M{sun}_) stars. Models incorporating magnetic inhibition of convection predict an overradius, but do not reproduce this mass dependence unless superequipartition surface magnetic fields are present at lower masses. Models incorporating flux blocking by starspots can explain the mass dependence but there is no evidence that spot coverage diminishes between the Pleiades and Praesepe samples to accompany the decline in overradius. The fastest rotating stars in both Praesepe and the Pleiades are significantly smaller than the slowest rotators for which a projected radius can be measured. This may be a selection effect caused by more efficient angular momentum loss in larger stars leading to their progressive exclusion from the analysed samples. Our analyses assume random spin-axis orientations; any alignment in Praesepe, as suggested by Kovacs, is strongly disfavoured by the broad distribution of projected radii.

Cone search capability for table J/MNRAS/483/1125/table2 (Properties of observed science targets in Praesepe)