Main sequence , fully-convective M dwarfs in eclipsing binaries are observed to be larger than stellar evolutionary models predict by as much as 10 - 15 \% .
A proposed explanation for this discrepancy involves effects from strong magnetic fields , induced by rapid-rotation via the dynamo process .
Although , a handful of single , slowly-rotating M dwarfs with radius measurements from interferometry also appear to be larger than models predict , suggesting that rotation or binarity specifically may not be the sole cause of the discrepancy .
We test whether single , rapidly rotating , fully convective stars are also larger than expected by measuring their R \sin i distribution .
We combine photometric rotation periods from the literature with rotational broadening ( v \sin i ) measurements reported in this work for a sample of 88 rapidly rotating M dwarf stars .
Using a Bayesian framework , we find that stellar evolutionary models underestimate the radii by 10 - 15 \% \substack { +3 \ -2.5 } , but that at higher masses ( 0.18 < M < 0.4 M _ { Sun } ) the discrepancy is only about 6 % and comparable to results from interferometry and eclipsing binaries .
At the lowest masses ( 0.08 < M < 0.18 M _ { Sun } ) , we find the discrepancy between observations and theory is 13 - 18 \% , and we argue that the discrepancy is unlikely to be due to effects from age .
Furthermore , we find no statistically significant radius discrepancy between our sample and the handful of M dwarfs with interferometric radii .
We conclude that neither rotation nor binarity is responsible for the inflated radii of fully convective M dwarfs , and that all fully-convective M dwarfs are larger than models predict .