We conducted interferometric observations with the CHARA Array of transiting super-Earth host HD 97658 and measured its limb-darkened angular diameter to be $theta_{text{LD}}=0.314pm0.004$ mas. The combination of the angular diameter with the Gaia EDR3 parallax value with zero-point correction ($pi=46.412pm0.022$ mas, $d=21.546pm0.011$ pc) yields a physical radius of $R_star=0.728pm0.008$ $R_odot$. We also measured the bolometric flux of the star to be $F_text{bol}=2.42pm 0.05times 10^{-8}$erg s$^{-1}$ cm$^{-2}$, which, together with angular size, allows a measurement of the effective temperature $T_{text{eff}}=5212pm43$ K. Our directly determined physical stellar properties are in good agreement with previous estimates derived from spectroscopy. We used our measurements in combination with stellar evolutionary models and properties of the transit of HD 97658 b to determine the mass and age of HD 97658 as well as constrain the properties of the planet. Our results and our analysis of the TESS lightcurve on the planet (TOI-1821) corroborate previous studies of this system with tighter uncertainties.
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