  We aim to use microlensing taking place in the lensed quasar Q2237+0305 to
study the structure of the broad line region and measure the size of the region
emitting the CIV and CIII] lines.
  Methods: Based on 39 spectrophotometric monitoring data points obtained
between Oct. 2004 and Dec. 2007, we derived lightcurves for the CIV and CIII]
emission lines. We used three different techniques to analyse the microlensing
signal. Different components of the lines (narrow, broad and very broad) are
identified and studied. We built a library of simulated microlensing
lightcurves that reproduce the signal observed in the continuum and in the
lines provided only the source size is changed. A Bayesian analysis scheme is
then developed to derive the size of the various components of the BLR.
  Results: 1. The half-light radius of the region emitting the CIV line is
found to be R_CIV ~ 66^{+110}_{-46} lt-days = 0.06$^{+0.09}_{-0.04}$ pc =
1.7$^{+2.8}_{-1.1}$\,10$^{17}$ cm (at 68.3% CI). Similar values are obtained
for CIII]. Relative sizes of the carbon-line and V-band continuum
emitting-regions are also derived with median values of R(line)/R(cont) in the
range 4 to 29, depending of the FWHM of the line component.
  2. The size of the CIV emitting region agrees with the Radius-Luminosity
relationship derived from reverberation mapping. Using the virial theorem we
derive the mass of the black hole in Q2237+0305 to be M_BH ~ 10^{8.3+/-0.3}
M_sun.
  3. We find that the CIV and CIII] lines are produced in at least 2 spatially
distinct regions, the most compact one giving rise to the broadest component of
the line. The broad and narrow line profiles are slightly different for CIV and
CIII].
  4. Our analysis suggests a different structure for the CIV and FeII+III
emitting regions, with the latter produced in the inner part of the BLR or in a
less extended emitting region than CIV.
