  For static black holes in Einstein gravity, if matter fields satisfy a few
general conditions, we conjecture that three characteristic parameters about
the spatial size of black holes, namely the outermost photon sphere area
$A_{\mathrm{ph,out}}$, the corresponding shadow area $A_{\mathrm{sh,out}}$ and
the horizon area $A_{\mathcal{H}}$ satisfy a series of universal inequalities
$9A_{\mathcal{H}}/4\leq A_{\mathrm{ph,out}}\leq A_{\mathrm{sh,out}}/3\leq 36\pi
M^2$, where $M$ is the ADM mass. We present a complete proof in the spherically
symmetric case and some pieces of evidence to support it in general static
cases. We also discuss the properties of the photon spheres in general static
spacetimes and show that, similar to horizon, photon spheres are also conformal
invariant structures of the spacetimes.
