We modify Larson's lattice model [J. Chem. Phys. 83 (1985) 2411] and use it to study micellar behavior in supercritical solvent-surfactant systems by large-scale Monte Carlo (MC) simulations. Carbon dioxide and perfluoroalkylpoly(ethylene oxide) serve as prototypes for the solvent and surfactant, respectively. Larson-model type parameters for carbon dioxide and perfluoroalkylpoly(ethylene oxide) are obtained using experimental values for critical parameters and solubility along with a modified Berthelot mixing rule. We perform canonical MC simulations at a supercritical temperature varying the structure of the surfactant, the solvent density and the surfactant concentration. Various properties such as the critical micelle concentration, the overlap concentration, the aggregate size distribution, and the size and shape of the micelles are calculated. The implications of these results for the thermodynamics of micellar formation in supercritical solvents are examined.