  Centaurus A (Cen A) is a bright radio source associated with the nearby
galaxy NGC 5128 where high-resolution radio observations can probe the jet at
scales of less than a light-day. The South Pole Telescope (SPT) and the Atacama
Pathfinder Experiment (APEX) performed a single-baseline very-long-baseline
interferometry (VLBI) observation of Cen A in January 2015 as part of VLBI
receiver deployment for the SPT. We measure the correlated flux density of Cen
A at a wavelength of 1.4 mm on a $\sim$7000 km (5 G$\lambda$) baseline.
Ascribing this correlated flux density to the core, and with the use of a
contemporaneous short-baseline flux density from a Submillimeter Array
observation, we infer a core brightness temperature of $1.4 \times 10^{11}$ K.
This is close to the equipartition brightness temperature, where the magnetic
and relativistic particle energy densities are equal. Under the assumption of a
circular Gaussian core component, we derive an upper limit to the core size
$\phi = 34.0 \pm 1.8~\mu\textrm{as}$, corresponding to 120 Schwarzschild radii
for a black hole mass of $5.5 \times 10^7 M_{\odot}$.
