  We consider the size distribution of superbubbles in a star forming galaxy.
Previous studies have tried to explain the distribution by using adiabatic
self-similar evolution of wind driven bubbles, assuming that bubbles stall when
pressure equilibrium is reached. We show, with the help of hydrodynamical
numerical simulations, that this assumption is not valid. We also include
radiative cooling of shells. In order to take into account non-thermal pressure
in the ambient medium, we assume an equivalent higher temperature than implied
by thermal pressure alone. Assuming that bubbles stall when the outer shock
speed becomes comparable to the ambient sound speed (which includes non-thermal
components), we recover the size distribution with a slope of $\sim -2.7$ for
typical values of ISM pressure in Milky Way, which is consistent with
observations. Our simulations also allow us to follow the evolution of size
distribution in the case of different values of non-thermal pressure, and we
show that the size distribution steepens with lower pressure, to slopes
intermediate between only-growing and only-stalled cases.
