  The supermassive black hole candidate at the center of M87 drives an
ultra-relativistic jet visible on kiloparsec scales, and its large mass and
relative proximity allow for event horizon scale imaging with very long
baseline interferometry at millimeter wavelengths (mm-VLBI). Recently,
relativistic magneto-hydrodynamic (MHD) simulations of black hole accretion
flows have proven capable of launching magnetically-dominated jets. We
construct time-dependent disc/jet models of the innermost portion of the M87
nucleus by performing relativistic radiative transfer calculations from one
such simulation. We identify two types of models, jet-dominated or disc/jet,
that can explain the spectral properties of M87, and use them to make
predictions for current and future mm-VLBI observations. The Gaussian source
size for the favored sky orientation and inclination from observations of the
large-scale jet is 33-44 microarcseconds (~4-6 Schwarzschild radii) on current
mm-VLBI telescopes, very similar to existing observations of Sgr A*. The black
hole shadow, direct evidence of an event horizon, should be visible in future
measurements using baselines between Hawaii and Mexico. Both models exhibit
variability at millimeter wavelengths with factor of ~2 amplitudes on year
timescales. For the low inclination of M87, the counter-jet dominates the event
horizon scale millimeter wavelength emission from the jet-forming region.
