  We derive a theoretical relation between $R_{BLR}$, the size of the
broad-emission-line region of active galactic nuclei, and the observed soft
X-ray luminosity and spectrum. We show that in addition to the well known
$R_{BLR}\sim L^{1/2}$ scaling, $R_{BLR}$ should depend also on the soft X-ray
spectral slope and derive the expected relation between $R_{BLR}$ and the X-ray
luminosity and spectral index. Applying this relation to calculate a predicted
BLR radius for ten AGN with reverberation data, we show that including the
dependence on the spectrum improves the agreement between the calculated BLR
radius and the radius independently determined from reverberation mapping.
Similarly, we evaluate an expression for the line width, and show that
including the dependence on the spectrum significantly improves the agreement
between the calculated BLR velocity dispersion and the observed FWHM of the
H$\beta$ line.
