Non-covalent immobilisation of catalysts in nanoporous materials is a promising way to apply homogeneous catalysts to heterogeneous catalytic reactions. Herein we report a size-specific catalytic reaction with an acid catalyst, p-toluenesulfonic acid, immobilised in a porous molecular crystal, metal–macrocycle framework (MMF), composed of metallo-macrocycles. A tritylated substrate which is smaller than the pore dimension of MMF was deprotected by the heterogeneous catalyst, whereas the reaction with a larger substrate was completely suppressed due to the steric restriction.