  We determine the absolute dimensions of the eclipsing binary V578 Mon, a
detached system of two early B-type stars (B0V + B1V, P$=$2.40848 d) in the
star-forming region NGC 2244 of the Rosette Nebula. From the light curve
analysis of 40 yr of photometry and the analysis of HERMES spectra, we find
radii of $5.41\pm0.04$ Rsun and $4.29\pm 0.05$ Rsun, and temperatures of $
30000\pm 500$~K and $ 25750\pm 435$ K respectively. We find that our
disentangled component spectra for V578 Mon agree well previous spectral
disentangling from the literature. We also reconfirm the previous spectroscopic
orbit of V578 Mon finding that masses of $ 14.54\pm 0.08$ Msun and $ 10.29\pm
0.06$ Msun are fully compatible with the new analysis. We compare the absolute
dimensions to the rotating models of the Geneva and Utrecht groups and the
models of Granada group. We find all three sets of models marginally reproduce
the absolute dimensions of both stars with a common age within uncertainty for
gravity-effective temperature isochrones. However - there are some apparent age
discrepancies for the corresponding mass-radius isochrones. Models with larger
convective overshoot $>0.35$ worked best. Combined with our previously
determined apsidal motion of $0.07089^{+0.00021}_{-0.00013}$ deg cycle$^{-1}$,
we compute the internal structure constants (tidal Love number) for the
newtonian and general relativistic contribution to the apsidal motion,
$\log{k_2}=-1.975\pm0.017$ and $\log{k_2}=-3.412\pm0.018$ respectively. We find
the relativistic contribution to the apsidal motion of be small $<4\%$. We find
that the prediction of $\log{k_{\rm 2,theo}}=-2.005\pm0.025$ of the Granada
models fully agrees with our observed $\log{k_2}$.
