  We present a model that predicts the light curve amplitude distribution for
an ensemble of low-mass magnetically active stars, under the assumptions that
stellar spin axes are randomly orientated and that cool starspots have a
characteristic scale length and are randomly distributed across the stellar
surfaces. The model is compared with observational data for highly magnetically
active M-dwarfs in the young cluster NGC 2516. We find that the best fitting
starspot scale length is not constrained by these data alone, but requires
assumptions about the overall starspot filling factor and starspot temperature.
Assuming a spot coverage fraction of 0.4+/-0.1 and a starspot to unspotted
photosphere temperature ratio of 0.7+/-0.05, as suggested by the inflated radii
of these stars compared to evolutionary model predictions and by TiO band
measurements on other active cool stars of earlier spectral type, the
best-fitting starspot angular scale length is 3.5(+2)(-1) degrees, or a linear
scale length of \sim 25000 km. This linear scale length is similar to large
sunspot groups, but 2--5 times smaller than the starspots recently deduced on
an active G-dwarf using eclipse mapping by a transiting exoplanet. However, the
best-fitting spot scale length in the NGC 2516 M-dwarfs increases with the
assumed spot temperature ratio and with the inverse square root of the assumed
spot filling factor. Hence the light curve amplitude distribution might equally
well be described by these larger spot scale lengths if the spot filling
factors are <0.1 or the spot temperature ratio is >0.9.
