The size dependence of the order-disorder transition in FePt nanoparticles with an L10 structure is investigated by means of Monte Carlo simulations based on an analytic bond-order potential for FePt. A cross parametrization for the Fe-Pt interaction is proposed, which complements existing potentials for the constituents Fe and Pt. This FePt potential properly describes structural properties of ordered and disordered phases, surface energies, and the L10 to A1 transition temperature in bulk FePt. The potential is applied for examining the ordering behavior in small particles. The observed lowering of the order-disorder transition temperature with decreasing particle size confirms previous lattice-based Monte Carlo simulations [M. Müller and K. Albe, Phys. Rev. B 72, 094203 (2005)]. Although a distinctly higher amount of surface induced disorder is found in comparison to previous studies based on lattice-type Hamiltonians, the presence of lattice strain caused by the tetragonal distortion of the L10 structure does not have a significant influence on the depression of the ordering temperature with decreasing particle size.