  From the peak of a gravitational microlensing high-magnification event in the
A component of QSO 2237+0305, which was accurately monitored by the GLITP
collaboration, we derived new information on the nature and size of the optical
V-band and R-band sources in the far quasar. If the microlensing peak is caused
by a microcaustic crossing, we firstly obtained that the standard accretion
disk is a scenario more reliable/feasible than other usual axially symmetric
models. Moreover, the standard scenario fits both the V-band and R-band
observations with reduced chi-square values very close to one. Taking into
account all these results, a standard accretion disk around a supermassive
black hole is a good candidate to be the optical continuum main source in QSO
2237+0305. Secondly, using the standard source model and a robust upper limit
on the transverse galactic velocity, we inferred that 90 per cent of the V-band
and R-band luminosities are emitted from a region with radial size less than
1.2 10^{-2} pc (= 3.7 10^{16} cm, at 2*sigma confidence level).
