  We quantitatively assess, by means of comprehensive numerical simulations,
the ability of broad-band photometric surveys to recover the broad emission
line region (BLR) size in quasars under various observing conditions and for a
wide range of object properties. Focusing on the general characteristics of the
Large Synoptic Survey Telescope (LSST), we find that the slope of the
size-luminosity relation for the BLR in quasars can be determined with
unprecedented accuracy, of order a few percent, over a broad luminosity range
and out to $z\sim 3$. In particular, major emission lines for which the BLR
size can be reliably measured with LSST include H$\alpha$, MgII $\lambda 2799$,
CIII] $\lambda 1909$, CIV $\lambda 1549$, and Ly$\alpha$, amounting to a total
of $\gtrsim 10^5$ time-delay measurements for all transitions. Combined with an
estimate for the emission line velocity dispersion, upcoming photometric
surveys will facilitate the estimation of black hole masses in AGN over a broad
range of luminosities and redshifts, allow for refined calibrations of BLR
size-luminosity-redshift relations in different transitions, as well as lead to
more reliable cross-calibration with other black hole mass estimation
techniques.
