This study characterized four α-synuclein isoforms — full-length αSyn and the splice variants αSynΔ3, αSynΔ5, and αSynΔ3Δ5 — to determine how differences in monomer conformation drive differences in amyloid formation kinetics relevant to Parkinson's disease and related synucleinopathies. Flow Induced Dispersion Analysis (FIDA), on a Fida-1 instrument, was used to measure the hydrodynamic radius of each variant across a range of ionic strengths, revealing that full-length αSyn and αSynΔ3 compact substantially at low ionic strength while αSynΔ5 and αSynΔ3Δ5 remain largely expanded. This exon 5-dependent compaction correlated with slower rates of secondary nucleation, linking monomer conformational state directly to amyloid propensity. It is an example of using FIDA to detect subtle, ionic strength-dependent conformational changes in an intrinsically disordered protein that govern downstream aggregation behavior.