Several observations suggest that the molecular cloud toward HD 210121 contains an enhanced relative abundance of small dust grains. In particular, the value of the ratio of total-to-selective extinction (Rv) is unusually low. In this paper, we estimate the size distribution of dust grains in this line of sight from the extinction curve observed in the near-infrared through the ultraviolet. We use the maximum entropy method (MEM) to find the smoothest possible size distribution consistent within a χ2 confidence level fit to the extinction data. While MEM has been shown to be a powerful tool in modeling average extinction curves, we show in this paper that MEM can also be a useful technique in modeling individual lines of sight that differ significantly from Galactic averages. We review existing data for HD 210121 and calculate the dust-to-gas ratio in order to constrain the amount of material used in the grain model. We present size distributions for both two-component models of silicate and graphite and three-component models that include amorphous carbon. We compare HD 210121 with the average diffuse interstellar medium and with three other high-latitude lines of sight. The grain-size distribution toward HD 210121 contains a relative excess of grains with radius ɑ< 0.1 µm as well as a relative deficiency of grains with radius ɑ > 0.1 µm as compared with the average diffuse interstellar medium and other clouds at high latitude.