The spin-flop transition of acicular hematite nanoparticles of different size synthesized by chemical route and annealed at different temperatures has been investigated as a function of temperature. Transmission electron microscopy (TEM) measurements show that particles have an ellipsoidal shape, with a major axis of $330\pm 50\mathrm{nm}$ and a minor axis of $70\pm 10\mathrm{nm}.$ TEM and x-ray diffraction experiments show that nanoparticles are made of hematite crystallites, which grow on increasing the annealing temperature. Both the Morin transition temperature ${(T}_M)$ and the spin-flop transition field ${(H}_{\mathrm{sf}})$ have been found to increase for increasing crystallite size $\mathrm{}\left(d\right):$ for instance, for $d=36\mathrm{}\mathrm{nm}$ particles $T_M=164\mathrm{}\mathrm{K}$ and the value of $H_{\mathrm{sf}}$ extrapolated at $T=0\mathrm{}$ ${(H}_{\mathrm{sf}0\mathrm{}})$ is 1.7 T, whereas for bulk hematite $T_M=263\mathrm{}\mathrm{K}$ and $H_{\mathrm{sf}0\mathrm{}}=6.5\mathrm{}\mathrm{T}.$ Both $H_{\mathrm{sf}0\mathrm{}}$ and $T_M$ follow a $1/d$ dependence (at a faster rate for $H_{\mathrm{sf}0\mathrm{}}),$ indicating that their variation is mainly driven by surface effects.