VLBI observations at 2.3 GHz of SN1987A on 28 February 1987 yielded no fringes, implying, for an optically thin shell, a lower bound on the (outer) diameter of 1.9 mas. From the comparison of the VLBI and optical results, we infer that the radiosphere of SN1987A was either about equal to, or larger than, the photosphere of the supernova five days after the explosion.
Shapiro, I., Bartel, N., Preston, R. A., Jones, D., Kemball, A., Nicolson, G. D., Jauncey, D., Reynolds, J. E., Whitney, A. R., Rogers, A. E. E., Phillips, R. B., Clark, T. A., & Robertson, D. (1988). VLBI Observations of Supernova 1987A. Symposium - International Astronomical Union, 129, 185-186. https://doi.org/10.1017/S0074180900134436