Nova V1974 Cygni (Nova Cygni 1992) was observed by ROSAT as a very bright soft X-ray source, with its point source image surrounded by an extensive halo caused by scattering from interstellar grains. We have analyzed the halo of 1992 December 6, using the standard extinction law (R = 3.1) as an additional constraint on the size distribution of the grains. We considered composite grains (with silicates and amorphous or hydrogenated carbon intermixed), possibly containing vacuum within the grains. We also included small graphite grains to provide the λ2175 bump.A critical parameter is the interstellar extinction between us and the nova. We estimate the reddening E(B- V) to be in the range 0.19-0.31 mag, with the low reddening suggested by the Hα/Hβ ratio as determined at the Pine Bluff Observatory. For these reddenings, composite grains can provide a good fit to both the X-ray halo and optical/ultraviolet extinction, provided that the fraction of vacuum is ≥ 25% for the larger grains in the distribution (a ≥ 0.1 μm). If E(B- V) = 0.3, as suggested by the He II λ4686/λ1640 ratio, the fit is slightly worse than for low reddening. The difficulty in fitting is providing enough extinction per H atom in the optical without producing too strong a halo. The halo of Nova Cygni 1992 suggests that interstellar grains contain a substantial fraction of vacuum. Probably the greatest uncertainty in this conclusion arises from the calculation of the optical properties of the composite grains. We cannot fit the observations with all types of carbon. Hydrogenated amorphous carbon and (to a lesser extent) "organic refractory" material fail to provide enough optical extinction and the proper slope to the X-ray halo. Every reasonable model has a maximum in the mass distribution a4n(a) (the mass per interval of log a) at a ≍ 0.15-0.3 μm, with the size increasing with the fraction of vacuum. Neither the halo nor the extinction law is a suitable diagnostic for the numbers of small grains (a <0.02 μm). The timing delays of the X-rays from Nova Cygni 1992 are too short (a few days) to be suitable for being a second diagnostic of the size distribution.