  One of the key observables of the reionization era is the distribution of
neutral and ionized gas. Recently, Furlanetto, Zaldarriaga, & Hernquist
developed a simple analytic model to describe the growth of HII regions during
this era. Here, we examine some of the fundamental simplifying assumptions
behind this model and generalise it in several important ways. The model
predicts that the ionized regions attain a well-defined characteristic size R_c
that ranges from R_c~1 Mpc in the early phases to R_c>10 Mpc in the late
phases. We show that R_c is determined primarily by the bias of the galaxies
driving reionization; hence measurements of this scale constrain a fundamental
property of the first galaxies. The variance around R_c, on the other hand, is
determined primarily by the underlying matter power spectrum. We then show that
increasing the ionizing efficiency of massive galaxies shifts R_c to
significantly larger scales and decreases the importance of recombinations.
These differences can be observed with forthcoming redshifted 21 cm surveys
(increasing the brightness temperature fluctuations by up to a factor of two on
large scales) and with measurements of small-scale anisotropies in the cosmic
microwave background. Finally, we show that stochastic fluctuations in the
galaxy population only broaden the bubble size distribution significantly if
massive galaxies are responsible for most of the ionizing photons. We argue
that the key results of this model are robust to many of our uncertainties
about the reionization process.
