A polarized absorption spectroscopy (PAS) study of molecular aggregate solutions of phthalocyanine molecules in high magnetic fields is presented. We show that a magnetic field can induce macroscopic ordering of molecular aggregates (MAs), in a clean, non-contact method, which is well suited for MA manipulation. We also demonstrate that magnetic-field-induced alignment in combination with PAS can be used to determine the MA size and the precise position of the absorption lines in concentrated solutions, as well as probe interaggregate interactions.