  High-redshift quasars (z >~ 6) drive ionization fronts into the intergalactic
medium (IGM). If the thickness of the front can be measured, it can provide a
novel constraint on the ionizing spectral energy distribution (SED). Here we
follow the propagation of an I-front into a uniform IGM, and compute its
thickness for a range of possible quasar spectra and ages. We also explore the
effects of uniform and non-uniform ionizing backgrounds. We find that even for
hard spectra, the fronts are initially thin, with a thickness much smaller than
the mean free path of ionizing photons, but the thickness increases as the
front approaches equilibrium in 10^8 - 10^9 years, and can eventually
significantly exceed simple estimates based on the mean free path. With a high
intrinsic hydrogen column density obscuring the source (log(N_H/cm^-2) >~ 19.2)
or a hard power-law spectrum combined with some obscuration (e.g.
dlog(F_\nu)/dlog(\nu) >~ -1.2 at log(N_H/cm^-2) >~ 18.0), the thickness of the
front exceeds ~1 physical Mpc and may be measurable from the morphology of its
redshifted 21cm signal. We find that the highly ionized inner part of the
front, which may be probed by Lyman line absorption spectra, remains sharp for
bright quasars unless a large obscuring column (log(N_H/cm^-2) >~ 19.2) removes
most of their ionizing photons up to ~40 eV. For obscured sources with
log(N_H/cm^-2) >~ 19.8, embedded in a significantly neutral IGM, the black
Lyman-alpha trough (where the neutral fraction is ~10^-3) underestimates the
size of the HII region by a factor of >~4.
