Protein-protein interactions are the fundamental driving force of numerous cellular processes and cell signaling pathways. Characterizing whether proteins interact as dimers, trimers, or higher oligomers is essential to understanding these interactions. Several microscopy and advanced imaging techniques relying on Forster resonance energy transfer (FRET) between identical fluorophores (homo-FRET) have been developed to estimate protein stoichiometry. The increased FRET in oligomers is detected by measuring depolarization or emission time. Homo-FRET methods have a strong advantage in requiring only a single fluorophore, greatly simplifying sample preparation in comparison to conventional hetero-FRET methods. However, most homo-FRET methods require sophisticated imaging equipment, and both theoretical models and applications have been restricted to the study of membrane-bound proteins. Using a simple bulk homo-FRET and laser photobleaching approach, we demonstrate the feasibility of characterizing the oligomerization state of an interacting protein in-vitro. To simulate oligomers in a proof of concept, we constructed an extensive repertoire of fusion proteins with 1-6 consecutive green fluorescent protein (GFP) domains. We show how the resulting homo-FRET (measurable via steady-state anisotropy or fluorescence polarization) is proportional to the oligomerization state of proximal GFP domains. For the first time, this is demonstrated with soluble proteins. In both membrane and soluble proteins, oligomerization increases FRET and therefore anisotropy. However for soluble proteins oligomerization also slows fluorophore rotation, leading to a size-dependent decrease in anisotropy. Through gradual photobleaching of fluorophores these two effects can be distinguished, and the oligomerization state of a labeled protein of interest can be estimated. We therefore show how the theoretical framework developed for membrane proteins needs to be adjusted to account for this additional degree of freedom in soluble proteins. Overall, bulk homo-FRET and laser photobleaching is a promising method to determine the oligomerization state of a protein of interest, which can have a low concentration (0.1-0.5 µM) and needs only a single fluorescent label. The method requires only a photometer or microplate reader capable of measuring steady-state anisotropy.