New-generation density functionals (M06-L and M06-2X) include an accurate treatment of medium-range correlation energy and have been applied to investigate host-guest interactions in supramolecular complexes in which a hydrocarbon nanoring, paraphenyleneacetylene (CPPA), acts as the host molecule. Guests include fullerenes and carbon nanotubes. The nature of the interactions has been discussed and analyzed. The size-selective supramolecular chemistry in the nanoring has been investigated by varying the size of the guest molecules and optimizing inclusion structures as large as C128H44. We found that the (5,5) armchair-type nanotube fits in the CPPA hydrocarbon nanoring better than the (3,3) or (4,4) ones, and C70 is bound more strongly than C60. The predicted host-guest binding energies of the (4,4), (5,5), C60, and C70 structures are 24, 43, 25, and 28 kcal/mol, respectively.