  We discuss a method for determination of the size of the emitting region
close to the compact star in a binary system with eclipses by a secondary,
which fills its Roche lobe. The often used approach is to model the Roche lobe
by a sphere with the "effective radius" corresponding to the volume of the
Roche lobe. This approach leads to a 4-6% overestimate of the radius, if taking
into account the angular dimensions of the Roche lobe seen form the compact
star. Andronov (1992) had shown that the projection of the Roche lobe onto the
celestial sphere is close to an ellipse and had tabulated these dimensions as a
function of the mass ratio. Also he published the coefficients of the
approximation similar to that of the Eggleton (1983) for the "sphere
corresponding to the same volume". We compare results obtained for the
"circle+circle", "ellipse+circle" and "ellipse+point" approximations of the
projections of the red dwarf and a white dwarf, respectively. Results are
applied to the recently discovered eclipsing polar CSS 081231:071126+440405.
