  We dendrogram the Leike et al. 2020 3D dust map, leveraging its $\sim 1$ pc
spatial resolution to produce a uniform catalog of molecular clouds in the
solar neighborhood. Using accurate distances, we measure the properties of 65
clouds in true 3D space, eliminating much of the uncertainty in mass, size, and
density. Clouds in the catalog contain a total of $1.1 \times 10^5 \; \rm
M_{\odot}$, span distances of $116-440$ pc, and include a dozen well-studied
clouds in the literature. In addition to deriving cloud properties in 3D volume
density space, we create 2D dust extinction maps from the 3D data by projecting
the 3D clouds onto a 2D "Sky" view. We measure the properties of the 2D clouds
separately from the 3D clouds. We compare the scaling relation between the
masses and sizes of clouds following Larson 1981. We find that our 2D projected
mass-size relation, $M \propto r^{2.1}$, agrees with Larson's Third Relation,
but our 3D derived properties lead to a scaling relation of about one order
larger: $M \propto r^{2.9}$. Validating predictions from theory and numerical
simulations, our results indicate that the mass-size relation is sensitive to
whether column or volume density is used to define clouds, since mass scales
with area in 2D ($M \propto r^{2}$) and with volume in 3D ($M \propto r^{3}$).
Our results imply a roughly constant column and volume density in 2D and 3D,
respectively, for molecular clouds, as would be expected for clouds where the
lower density, larger volume-filling gas dominates the cloud mass budget.
