  The size of a galaxy is one of the fundamental parameters that reflects its
growth and assembly history. Traditionally, the size of the Milky Way has been
characterized by the scale length of the disk, based on the assumption of an
exponential density profile. Earlier scale length measurements suggest the
Milky Way is an overly compact galaxy, compared to similar galaxies of its
mass. These size measurements, however, ignore the presence of the bulge, and
the assumption of a single-exponential disk profile faces growing challenges
from the recent observations. The half-light radius is an alternative size
measurement that is independent of the galaxy density profile and has been
widely used to quantify the size of external galaxies. Here we report the
half-light radius of the Milky Way, derived from a new measurement of the
age-resolved Galactic surface brightness profile in an unprecedentedly wide
radial range from ${\rm R=0}$ to 17~kpc. We find a broken surface brightness
profile with a nearly flat distribution between 3.5 and 7.5 kpc, which results
in a half-light radius of 5.75$\pm$0.38 kpc, significantly larger than the
scale-length inferred from the canonical single-exponential disk profile but in
good consistency with local disk galaxies of similar mass. Because our density
profile can be decomposed by stellar age and extrapolated backwards in time, we
can also confirm that the size history of the Milky Way is broadly consistent
with high-redshift galaxies but with systematically smaller size at each look
back time. Our results suggest that the Milky Way is a typical disk galaxy
regarding its size and has likely experienced inefficient secular size growth.
