Precise control over the porosity of metal-organic frameworks (MOFs) is crucial to optimize their properties and leverage their inherent tunability. However, there are ongoing challenges in pore size engineering for each MOF platform such as preserving crystallinity and morphology and facilitating reliable theoretical predictions throughout a series of modulated structures. Among postsynthetic strategies, mainly covalent functionalization appears to simultaneously preserve structural integrity and enable accurate theoretical predictions. Here, we present a MOF platform [M₂(RCOO)₄(H₂O)₂], JUK-21(M), M = Cu or Zn, containing a tetrazine-based tetracarboxylate linker, which we covalently functionalize using the inverse electron-demand Diels-Alder reaction (iEDDA) and five dienophiles of various bulkiness, yielding a series of JUK-21(Cu)-x MOFs. In addition to experiments, the iEDDA reactivity is assessed by applying a charge distribution susceptibility analysis, including Fukui functions, hardness, and relevant donor/acceptor orbitals. Comprehensive theoretical and experimental insights into the adsorption of nitrogen and hydrogen by JUK-21(Cu)-x enable rationalization of the observed isotherms and show the isosteric heat of hydrogen adsorption as a highly sensitive parameter to validate the modification efficiency. Our findings indicate to what extent the pore size of MOFs affects the adsorption properties and highlight potential pitfalls that arise even with the precise covalent functionalization of MOFs.