  Despite the importance of their size evolution in understanding the dynamical
evolution of globular clusters (GCs) of the Milky Way, studies are rare that
focus specifically on this issue. Based on the advanced, realistic
Fokker-Planck (FP) approach, we predict theoretically the initial size
distribution (SD) of the Galactic GCs along with their initial mass function
and radial distribution. Over one thousand FP calculations in a wide parameter
space have pinpointed the best-fit initial conditions for the SD, mass
function, and radial distribution. Our best-fit model shows that the initial SD
of the Galactic GCs is of larger dispersion than today's SD, and that typical
projected half-light radius of the initial GCs is ~4.6 pc, which is 1.8 times
larger than that of the present-day GCs (~2.5 pc). Their large size signifies
greater susceptibility to the Galactic tides: the total mass of destroyed GCs
reaches 3-5x10^8 M_sun$, several times larger than the previous estimates. Our
result challenges a recent view that the Milky Way GCs were born compact on the
sub-pc scale, and rather implies that (1) the initial GCs are generally larger
than the typical size of the present-day GCs, (2) the initially large GCs
mostly shrink and/or disrupt as a result of the galactic tides, and (3) the
initially small GCs expand by two-body relaxation, and later shrink by the
galactic tides.
