  The distribution of planet sizes encodes details of planet formation and
evolution. We present the most precise planet size distribution to date based
on Gaia parallaxes, Kepler photometry, and spectroscopic temperatures from the
California-Kepler Survey. Previously, we measured stellar radii to 11%
precision using high-resolution spectroscopy; by adding Gaia astrometry, the
errors are now 2%. Planet radius measurements are, in turn, improved to 5%
precision. With a catalog of ~1000 planets with precise properties, we probed
in fine detail the gap in the planet size distribution that separates two
classes of small planets, rocky super-Earths and gas-dominated sub-Neptunes.
Our previous study and others suggested that the gap may be observationally
under-resolved and inherently flat-bottomed, with a band of forbidden planet
sizes. Analysis based on our new catalog refutes this; the gap is partially
filled in. Two other important factors that sculpt the distribution are a
planet's orbital distance and its host star mass, both of which are related to
a planet's X-ray/UV irradiation history. For lower mass stars, the bimodal
planet distribution shifts to smaller sizes, consistent with smaller stars
producing smaller planet cores. Details of the size distribution including the
extent of the `sub-Neptune desert' and the width and slope of the gap support
the view that photoevaporation of low-density atmospheres is the dominant
evolutionary determinant of the planet size distribution.
