WR 118 is a highly evolved Wolf-Rayet star of the WC10 subtype surrounded by a permanent dust shell absorbing and re-emitting in the infrared a considerable fraction of the stellar luminosity. We present the first diffraction-limited 2.13 μm speckle interferometric observations of WR 118 with 73 mas resolution. The speckle interferograms were obtained with the 6 m telescope at the Special Astrophysical Observatory. The two-dimensional visibility function of the object does not show any significant deviation from circular symmetry. The visibility curve declines towards the diffraction cut-off frequency to ~0.66 and can be approximated by a linear function. Radiative transfer calculations have been carried out to model the spectral energy distribution, given in the range of 0.5-25 μm, and our 2.13 μm visibility function, assuming spherical symmetry of the dust shell. Both can be fitted with a model containing double-sized grains ("small" and "large" ) with the radii of $a = 0.05$ μm and 0.38 μm, and a mass fraction of the large grains greater than 65% . Alternatively, a good match can be obtained with the grain size distribution function $n(a) \sim a^{-3}$, with a ranging between 0.005 μm and 0.6 μm. At the inner boundary of the modelled dust shell (angular diameter $\Theta_{\rm in} = (17 \pm 1)$ mas), the temperature of the smallest grains and the dust shell density are 1750 K ± 100 K and $(1 \pm 0.2) \times 10^{-19}$ g/cm3, respectively. The dust formation rate is found to be $(1.3 \pm 0.5) \times 10^{-7}$ $M_{\odot}$/yr, assuming $V_{\rm wind} = 1200$ km s-1.