  This work explores the relationships between galaxy sizes and related
observable galaxy properties in a large volume cosmological hydrodynamical
simulation. The objectives of this work are to both develop a better
understanding of the correlations between galaxy properties and the influence
of environment on galaxy physics in order to build an improved model for the
galaxy sizes, building off of the {\it fundamental plane}. With an accurate
intrinsic galaxy size predictor, the residuals in the observed galaxy sizes can
potentially be used for multiple cosmological applications, including making
measurements of galaxy velocities in spectroscopic samples, estimating the rate
of cosmic expansion, and constraining the uncertainties in the photometric
redshifts of galaxies. Using projection pursuit regression, the model
accurately predicts intrinsic galaxy sizes and have residuals which have
limited correlation with galaxy properties. The model decreases the spatial
correlation of galaxy size residuals by a factor of $\sim$ 5 at small scales
compared to the baseline correlation when the mean size is used as a predictor.
