The Day plot (Mrs/Ms vs Hcr/Hc) is widely used by paleomagnetists to estimate the size of ferromagnetic grains and classify them as single domain (SD), pseudo single domain (PSD) or multidomain (MD). How reliable is this plot? To find out, a numerical micromagnetic model is used to calculate hysteresis loops as a function of grain size for two grain shapes (cube and cuboid with X=1.5Y=1.4Z). Magnetocrystalline anisotropy is ignored. The average Mrs/Ms is calculated for a collection of randomly oriented grains: In the elongated grain, it drops from 0.4 to 0.06 over a negligible size range, almost missing the usual PSD range altogether. Other hysteresis parameters, Hc, Hcr and X0, can only be calculated for a grain at a time. This is done for two magnetic field directions (close to the longest axis and close to the shortest axis). The single-grain values of Hcr/Hc depend strongly on field direction, but it is clear that the average jumps rapidly from SD to MD values. There are large, rapid fluctuations in Hcr and X0 associated with changes in remanent states. However, these fluctuations may not be apparent when averaged over a broad size range. This may explain why Hcr and X0 depend weakly on grain size in real samples. Overall, it is predicted that hysteresis parameters do not represent a typical grain size. Instead, they depend strongly on the size distribution.