The co-conformational ensembles of three differently sized catenanes were studied by measuring pair correlation functions corresponding to the separation of nitroxide spin labels--one attached to each of the two macrocycles--with the double electron-electron resonance (DEER) experiment. A geometric model for the catenanes was derived that approximates the macrocycles by circles and takes into account the topological constraint. Comparison of the experimental to the theoretically predicted pair correlation functions gives insight into the co-conformational distribution and the size of the macrocycles. It was found that the macrocycles of the medium- and large-sized catenanes in chloroform are close to fully expanded, while they are partially collapsed in glassy o-terphenyl. For the small-sized catenane, moderate interaction between the unsaturated sections of the macrocycles in chloroform is indicated by a slight over-representation of short label-to-label separations in the pair correlation function.