According to the surveys carried out by the NEOWISE mission of NASA's Wide-field Infrared Survey Explorer (WISE), the Infrared Astronomical Satellite IRAS, and the Japanese Akari satellite, Petunia measures , and kilometers in diameter, and its surface has an albedo of , and , respectively. An additional measurement published by the WISE team gives an alternative mean-diameter of . The Collaborative Asteroid Lightcurve Link adopts the WISE results revised by Pravec, that is an albedo of 0.1654 and a diameter of 29.12 km based on an absolute magnitude of 10.25. An asteroid occultation on 30 September 2014, gave a best-fit ellipse dimension of (). These timed observations are taken when the asteroid passes in front of a distant star. However the quality of the measurements are poorly rated.
