  The radio emission from Sgr A$^\ast$ is thought to be powered by accretion
onto a supermassive black hole of $\sim\! 4\times10^6~ \rm{M}_\odot$ at the
Galactic Center. At millimeter wavelengths, Very Long Baseline Interferometry
(VLBI) observations can directly resolve the bright innermost accretion region
of Sgr A$^\ast$. Motivated by the addition of many sensitive, long baselines in
the north-south direction, we developed a full VLBI capability at the Large
Millimeter Telescope Alfonso Serrano (LMT). We successfully detected Sgr
A$^\ast$ at 3.5~mm with an array consisting of 6 Very Long Baseline Array
telescopes and the LMT. We model the source as an elliptical Gaussian
brightness distribution and estimate the scattered size and orientation of the
source from closure amplitude and self-calibration analysis, obtaining
consistent results between methods and epochs. We then use the known scattering
kernel to determine the intrinsic two dimensional source size at 3.5 mm:
$(147\pm7~\mu\rm{as}) \times (120\pm12~\mu\rm{as})$, at position angle
$88^\circ\pm7^\circ$ east of north. Finally, we detect non-zero closure phases
on some baseline triangles, but we show that these are consistent with being
introduced by refractive scattering in the interstellar medium and do not
require intrinsic source asymmetry to explain.
