  We present empirical relations for determining the amount by which the
effective temperatures and radii---and therefore the estimated masses---of
low-mass stars and brown dwarfs are altered due to chromospheric activity.
Accurate estimates of stellar radii are especially important in the context of
searches for transiting exoplanets, which rely upon the assumed stellar
radius/density to infer the planet radius/density. Our relations are based on a
large set of well studied low-mass stars in the field and on a set of benchmark
low-mass eclipsing binaries. The relations link the amount by which an active
object's temperature is suppressed, and its radius inflated, to the strength of
its Halpha emission. These relations are found to approximately preserve
bolometric luminosity. We apply these relations to the peculiar brown-dwarf
eclipsing binary 2M0535-05, in which the active, higher-mass brown dwarf has a
cooler temperature than its inactive, lower-mass companion. The relations
correctly reproduce the observed temperatures and radii of 2M0535-05 after
accounting for the Halpha emission; 2M0535-05 would be in precise agreement
with theoretical isochrones were it inactive. The relations that we present are
applicable to brown dwarfs and low-mass stars with masses below 0.8 Msun and
for which the activity, as measured by Halpha, is in the range -4.6 < log
Lha/Lbol < -3.3. We expect these relations to be most useful for correcting
radius and mass estimates of low-mass stars and brown dwarfs over their active
lifetimes (few Gyr). We also discuss the implications of this work for
determinations of young cluster IMFs.
