  We present qualitative but precise description of the linear beam-size effect
predicted for the processes in which unstable but long--living particles
collide with each other. We derive physically pronounced equation for the
events rate which proves that the linear beam-size effect corresponds to the
scattering of one beam of particles on the decay products of the other. We
compare this linear beam-size effect with the known logarithmic beam-size
effect measured in the experiments on a single bremsstrahlung at VEPP-4 and
HERA.
