This study engineered a bright single-cell fluorescent reporter (NGFP4) that enables ultrasensitive target detection in microbial whole-cell biosensors. Flow Induced Dispersion Analysis (FIDA) was used to measure the hydrodynamic radius and concentration-dependent aggregation state of the purified fluorescent proteins, comparing the values against the theoretical sizes of monomeric sfGFP and NGFP4 to confirm they remained largely monomeric rather than aggregated. It is an example of using FIDA to assess protein size, monodispersity and aggregation state in solution.