  The size of a disk encodes important information about its evolution.
Combining new Submillimeter Array (SMA) observations with archival Atacama
Large Millimeter Array (ALMA) data, we analyze mm continuum and CO emission
line sizes for a sample of 44 protoplanetary disks around stars with masses of
0.15--2\,$M_{\odot}$ in several nearby star-forming regions. Sizes measured
from $^{12}$CO line emission span from 50 to 1000\,au. This range could be
explained by viscous evolution models with different $\alpha$ values (mostly of
$10^{-4}-10^{-3}$) and/or a spread of initial conditions. The CO sizes for most
disks are also consistent with MHD wind models that directly remove disk
angular momentum, but very large initial disk sizes would be required to
account for the very extended CO disks in the sample. As no CO size evolution
is observed across stellar ages of 0.5--20\,Myr in this sample, determining the
dominant mechanism of disk evolution will require a more complete sample for
both younger and more evolved systems. We find that the CO emission is
universally more extended than the continuum emission by an average factor of
$2.9\pm1.2$. The ratio of the CO to continuum sizes does not show any trend
with stellar mass, mm continuum luminosity, or the properties of substructures.
The GO Tau disk has the most extended CO emission in this sample, with an
extreme CO to continuum size ratio of 7.6. Seven additional disks in the sample
show high size ratios ($\gtrsim4$) that we interpret as clear signs of
substantial radial drift.
