We constrain the rest-UV size-luminosity relation for star-forming galaxies at z similar to 4 and z similar to 6, 7, and 8 identified behind clusters from the Hubble Frontier Fields (HFF) program. The size-luminosity relation is key to deriving accurate luminosity functions (LF) for faint galaxies. Making use of the latest lensing models and full data set for these clusters, lensing-corrected sizes and luminosities are derived for 68 z similar to 4, 184 z similar to 6, 93 z similar to 7, and 53 z similar to 8 galaxies. We show that size measurements can be reliably measured up to linear magnifications of similar to 30x, where the lensing models are well calibrated. The sizes we measure span a >1 dex range, from