Dimeric Ln(3+)[15-metallacrown-5] compartments selectively recognize carboxylates through guest binding to host metal ions and intermolecular interactions with the phenyl side chains. A systematic study is presented on how the size, selectivity, and number of encapsulated guests in the dimeric containers is influenced by the Ln(3+)[15-metallacrown(Cu(II))-5] ligand side chain and central metal. Compartments of varying heights were assembled from metallacrowns with S-phenylglycine hydroxamic acid (pgHA), S-phenylalanine hydroxamic acid (pheHA), and S-homophenylalanine hydroxamic acid (hpheHA) ligands. Guests that were examined include the fully deprotonated forms of terephthalic acid, isonicotinic acid, and bithiophene dicarboxylic acid (btDC). X-ray crystallography reveals that the side-chain length constrains the maximum and minimum length guest that can be encapsulated in the compartment. Compartments with heights ranging from 9.7 to 15.2 angstrom are formed with different phenyl side chains that complex 4.3-9.2 angstrom long guests. Up to five guests are accommodated in Ln(3+)[15-metallacrown(Cu(II))-5] compartments depending on steric effects from the host side chains. The nine-coordinate La3+ central metal promotes the encapsulation of multiple guests, while the eight-coordinate Gd3+ typically binds only one dicarboxylate. Electrospray ionization mass spectrometry reveals that the dimerization phenomenon occurs beyond the solid state, suggesting that these containers can be utilized in solid-state and solution applications.