A simple and straightforward method combines microwave-assisted solvothermal conditions with the coordination modulation method to achieve the size-controlled formation of the porous coordination polymer (PCP), [Cu(3)(btc)(2)] (where btc represents benzene-1,3,5-tricarboxylate) in the nano/micro regimes. The addition of a monocarboxylic acid modulator to the reaction mixture greatly influenced the morphology of the resulting sample, through competitive coordination interactions during the crystal formation process. By adjusting the concentration of dodecanoic acid additive, we could subtly control the nucleation rate of a [Cu(3)(btc)(2)] framework and, thus, the resulting crystal size. Homogeneous nanocrystals of the PCP with sizes ranging from few tenths of nanometers up to few micrometers could be successfully obtained in a controlled manner. X-ray diffractions and gas sorption measurements revealed highly crystalline particles with large pore volumes. Moreover, variations in the sorption profiles could be correlated to the size and morphology of the [Cu(3)(btc)(2)] samples, presenting affinity for gas condensation at high relative pressures or even hierarchical dual porous structures with mesoporous grain boundaries.