We present the stellar mass-size relation for 49 galaxies within the z =1.067 cluster SPTCL J0546-5345, with full width at half-maximum similar to 80-120 mas K-s=band data from the Gemini multiconjugate adaptive optics system ( GeMS/GSAOI). This is the first such measurement in a cluster environment, performed at sub-kpc resolution at rest-frame wavelengths dominated by the light of the underlying old stellar populations. The observed stellarmass-size relation is offset from the local relation by 0.21 dex, corresponding to a size evolution proportional to ( 1 + z)(-1.25), consistent with the literature. The slope of the stellar mass-size relation beta = 0.74 +/- 0.06, consistent with the local relation. The absence of slope evolution indicates that the amount of size growth is constant with stellar mass. This suggests that galaxies in massive clusters such as SPT-CL J0546-5345 grow via processes that increase the size without significant morphological interference, such as minor mergers and/or adiabatic expansion. The slope of the cluster stellar mass-size relation is significantly shallower if measured in Hubble Space Telescope ( HST)/Advanced Camera for Surveys imaging at wavelengths blueward of the Balmer break, similar to rest-frame ultraviolet relations at z=1 in the literature. The stellar mass-size relation must be measured at redder wavelengths, which are more sensitive to the old stellar population that dominates the stellar mass of the galaxies. The slope is unchanged when GeMS Ks-band imaging is degraded to the resolution of K-band HST/Near Infrared Camera and Multi-Object Spectrometer resolution but dramatically affected when degraded to Ks-band Magellan/FourStar resolution. Such measurements must be made with adaptive optics in order to accurately characterize the sizes of compact, z = 1 galaxies.