This review examines how microfluidic techniques quantify biomolecular interactions — binding affinity and molecular size — directly in solution using very small samples. It focuses on Flow Induced Dispersion Analysis (FIDA), which combines Taylor dispersion analysis with fluorescence detection in fused-silica capillaries to measure how a labeled ligand's apparent hydrodynamic radius changes as it binds its target, yielding dissociation constants and complex sizes. FIDA can size species from 0.5 to 1000 nm, works in complex media like plasma or fermentation broth, and runs on dedicated commercial equipment (Fida 1 by Fidabio). The authors compare FIDA with microfluidic diffusional sizing, describe premix, capillary-mix and dissociation modes, and highlight applications in antibody, protein and lipid-vesicle binding. These methods complement structural techniques such as X-ray crystallography, cryo-EM and NMR.