I use photometric reverberation mapping to measure quasar accretion-disk sizes from interband delays and to test the disk size–luminosity link. For Rubin's Deep Drilling Fields I analyze simulated light curves with realistic cadences, assessing how broadline and diffuse-continuum leakage and survey sampling impact lag recovery and the standardization of quasars for cosmology. In parallel I lead a medium-band campaign with the MPG/ESO La Silla telescope on high-redshift quasars, tying disks to the broad-line region where CIV lags exist. I highlight a direct disk-size estimate for J0455-4216, consistent with a thin disk around a roughly billion-solar-mass black hole, and a target with a GRAVITY-plus BLR size that enables an interferometry–reverberation cross-check