  In young circumstellar disks, accretion--the inspiral of disk material onto
the central star--is important for both the buildup of stellar masses and the
outcome of planet formation. Although the existence of accretion is well
documented, understanding the angular momentum transport mechanism that enables
disk accretion has proven to be an enduring challenge. The leading theory to
date, the magnetorotational instability, which redistributes angular momentum
within the disk, is increasingly questioned, and magnetothermal disk winds,
which remove angular momentum from the disk, have emerged as an alternative
theoretical solution. Here we investigate whether measurements of disk radii
can provide useful insights into which, if either, of these mechanisms drive
disk accretion, by searching for evidence of viscous spreading in gaseous
disks, a potential signature of "in disk" angular momentum transport. We find
that the large sizes of most Class II (T Tauri) gas disks compared to those of
their earlier evolutionary counterparts, Class I gas disks, are consistent with
expectations for viscous spreading in the Class II phase. There is, however, a
large spread in the sizes of Class II gas disks at any age, including a
population of very small Class II gas disks. Their small sizes may result from
processes such as photoevaporation, disk winds, or truncation by orbiting low
mass companions.
