The galaxy cluster environment is known to be capable of altering the evolutionary paths of galaxies through a variety of mechanisms. High relative velocities of satellite galaxies make minor mergers between them rare. The presence of an intra-cluster medium makes it possible to rapidly quench star formation in satellite galaxies via gas stripping processes. Galaxy clusters are therefore excellent laboratories for testing which galaxy size growth mechanisms dominate and how the quiescent population of galaxies builds up with redshift. I will present results from the HST WFC3 and grism follow up to GCLASS - the largest spectroscopic survey conducted on 10 clusters at z~1. Along with a field sample of galaxies from the 3D-HST survey, I will demonstrate how minor merger suppression, Brightest Cluster Galaxy (BCG) growth, Intra-cluster light (ICL) growth and the production of recently quenched galaxies can work simultaneously to maintain a stellar mass—size relation that is independent of environment at low redshifts. Last but not least, I will present the first spatially resolved Hα maps of cluster galaxies at z~1, and what these have uncovered about environmental quenching at this crucial epoch in the history of cosmic star formation.