  We construct a Hertzsprung-Russell diagram for the stellar/substellar
boundary based on a sample of 63 objects ranging in spectral type from M6V to
L4. We report newly observed VRI photometry for all 63 objects and new
trigonometric parallaxes for 37 objects. The remaining 26 objects have
trigonometric parallaxes from the literature. We combine our optical photometry
and trigonometric parallaxes with 2MASS and WISE photometry and employ a novel
SED fitting algorithm to determine effective temperatures, bolometric
luminosities, and radii. Our uncertainties range from ~20K to ~150K in
temperature, ~0.01 to ~0.06 in log(L/Ls}) and 3% to 10% in radius. We check our
methodology by comparing our calculated radii to radii directly measured via
long baseline optical interferometry. We find evidence for the local minimum in
the radius-temperature and radius-luminosity trends that signals the end of the
stellar main sequence and the start of the brown dwarf sequence at T ~ 2075K,
log(L/Ls)~ -3.9, and (R/Rs) ~ 0.086. The existence of this local minimum is
predicted by evolutionary models, but at temperatures ~400K cooler. The minimum
radius happens near the locus of 2MASS J0523-1403, an L2.5 dwarf with V-K=9.42.
We make qualitative arguments as to why the effects of the recent revision in
solar abundances accounts for the discrepancy between our findings and the
evolutionary models. We also report new color-absolute magnitude relations for
optical and infrared colors useful for estimating photometric distances. We
study the optical variability of all 63 targets and find an overall variability
fraction of 36^{+9}_{-7}% at a threshold of 15 milli-magnitudes in the I band,
in agreement with previous studies.
