  We observed the rapid radio brightening of GRB 210702A with the Australian
Telescope Compact Array (ATCA) just 11hr post-burst, tracking early-time radio
variability over a 5hr period on ~15min timescales at 9.0, 16.7, and 21.2GHz. A
broken power-law fit to the 9.0GHz light curve showed that the 5hr flare peaked
at a flux density of 0.4+/-0.1mJy at ~13hr post-burst with a steep rise and
decline. The observed temporal and spectral evolution are not expected in the
standard internal-external shock model, where forward and reverse shock radio
emission evolves on much longer timescales. The early-time (<1day) optical and
X-ray light curves from the Neil Gehrels Swift Observatory demonstrated typical
afterglow forward shock behaviour, allowing us to use blast wave physics to
determine a likely homogeneous circumburst medium and an emitting electron
population power-law index of p=2.9+/-0.1. We suggest the early-time radio
flare is likely due to weak interstellar scintillation (ISS), which boosted the
radio afterglow emission above the ATCA sensitivity limit on minute timescales.
Using weak ISS relations, we were able to place an upper limit on the size of
the blast wave of $\leq6 \times 10^{16}$cm in the plane of the sky, which is
consistent with the theoretical forward shock size prediction of
$8\times10^{16}$cm for GRB 210702A at ~13h post-burst. This represents the
earliest ISS size constraint on a GRB blast wave to date, demonstrating the
importance of rapid (<1day) radio follow-up of GRBs using several-hour
integrations to capture the early afterglow evolution, and to track
scintillation over a broad frequency range.
