Context : Rapidly rotating , low-mass members of eclipsing binary systems have measured radii that are significantly larger than predicted by standard evolutionary models .
It has been proposed that magnetic activity is responsible for this radius inflation .

Aims : By estimating the radii of low-mass stars in three young clusters ( NGC 2264 , NGC 2547 , NGC 2516 , with ages of \sim 5 , \sim 35 and \sim 140 Myr respectively ) , we aim to establish whether similar radius inflation is seen in single , magnetically active stars .

Methods : We use radial velocities from the Gaia-ESO Survey ( GES ) and published photometry to establish cluster membership and then combine GES measurements of projected equatorial velocities with published rotation periods to estimate the average radii for groups of fast-rotating cluster members as a function of their luminosity and age .
The average radii are compared with the predictions of both standard evolutionary models and variants that include magnetic inhibition of convection and starspots .

Results : At a given luminosity , the stellar radii in NGC 2516 and NGC 2547 are larger than predicted by standard evolutionary models at the ages of these clusters .
The discrepancy is least pronounced and not significant ( \simeq 10 per cent ) in ZAMS stars with radiative cores , but more significant in lower-mass , fully convective pre main-sequence cluster members , reaching \simeq 30 \pm 10 per cent .
The uncertain age and distance of NGC 2264 preclude a reliable determination of any discrepancy for its members .

Conclusions : The median radii we have estimated for low-mass fully convective stars in the older clusters are inconsistent ( at the 2 - 3 \sigma level ) with non-magnetic evolutionary models and more consistent with models that incorporate the effects of magnetic fields or dark starspots .
The available models suggest this requires either surface magnetic fields exceeding 2.5 kG , spots that block about 30 per cent of the photospheric flux , or a more moderate combination of both .