  We analyze the Aquarius simulations to characterize the shape of dark matter
halos with peak circular velocity in the range 8<Vmax<200 km/s, and perform a
convergence study using the various Aquarius resolution levels. For the
converged objects, we determine the principal axis (a<b<c) of the normalized
inertia tensor as a function of radius. We find that the triaxiality of field
halos is an increasing function of halo mass, so that the smallest halos in our
sample are ~40-50% rounder than Milky Way-like objects at the radius where the
circular velocity peaks, rmax. We find that the distribution of subhalo axis
ratios is consistent with that of field halos of comparable Vmax. Inner and
outer contours within each object are well aligned, with the major axis
preferentially pointing in the radial direction for subhalos closest to the
center of their host halo. We also analyze the dynamical structure of subhalos
likely to host luminous satellites comparable to the classical dwarf
spheroidals in the Local Group. These halos have axis ratios that increase with
radius, and which are mildly triaxial with <b/a>~0.75 and <c/a>~0.60 at r~1
kpc. Their velocity ellipsoid become strongly tangentially biased in the
outskirts as a consequence of tidal stripping.
