Methane adsorption in coal nanopores is governed by the molecular structure of coal and by nanoscale confinement. In this study, an idealized coal slit-pore model was constructed to examine pore-size-dependent methane adsorption under simplified nanopore conditions. Slit pores of 0.5, 1, 2, and 3 nm were generated by inserting vacuum layers between coal molecular cells. The coal macromolecular structure was optimized by molecular mechanics and molecular dynamics annealing, and methane adsorption was then simulated by grand canonical Monte Carlo calculations at 298 K over a fugacity range of 0.01–10 MPa. The adsorption isotherms were compared with the Langmuir–Freundlich model to evaluate adsorption capacity and surface heterogeneity. The results show that methane uptake increases with fugacity and pore size. At 10 MPa, the absolute adsorption capacity increases from 118.9 to 176.5 ml g−1 as the pore size increases from 0.5 to 3 nm. The isosteric heat indicates progressive occupation of adsorption sites with different interaction strengths, and spatial adsorption maps show preferential occupation of near-surface high-affinity regions at low fugacity followed by gradual filling of pore-center regions at higher fugacity. These results provide molecular-level insight into confinement-controlled methane adsorption in idealized coal slit pores and clarify how pore size modifies adsorption capacity, adsorption energetics, and methane spatial distribution.