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PSR J0740$+$6620 has a gravitational mass of $2.08\pm 0.07~M_\odot$, which isthe highest reliably determined mass of any neutron star. As a result, ameasurement of its radius will provide unique insight into the properties ofneutron star core matter at high densities. Here we report a radius measurementbased on fits of rotating hot spot patterns to Neutron Star InteriorComposition Explorer (NICER) and X-ray Multi-Mirror (XMM-Newton) X-rayobservations. We find that the equatorial circumferential radius of PSRJ0740$+$6620 is $13.7^{+2.6}_{-1.5}$ km (68%). We apply our measurement,combined with the previous NICER mass and radius measurement of PSRJ0030$+$0451, the masses of two other $\sim 2~M_\odot$ pulsars, and the tidaldeformability constraints from two gravitational wave events, to threedifferent frameworks for equation of state modeling, and find consistentresults at $\sim 1.5-3$ times nuclear saturation density. For a givenframework, when all measurements are included the radius of a $1.4~M_\odot$neutron star is known to $\pm 4$% (68% credibility) and the radius of a$2.08~M_\odot$ neutron star is known to $\pm 5$%. The full radius range thatspans the $\pm 1\sigma$ credible intervals of all the radius estimates in thethree frameworks is $12.45\pm 0.65$ km for a $1.4~M_\odot$ neutron star and$12.35\pm 0.75$ km for a $2.08~M_\odot$ neutron star.

The Radius of PSR J0740+6620 from NICER and XMM-Newton Data