Polarization switching in ferroelectric BaTiO3 can proceed through fundamentally different mechanisms - yet the conditions that determine which pathway is realized remain poorly understood. Using machine-learning potential-based molecular dynamics with the MACEField model, we systematically vary supercell size to reveal a clear transition from homogeneous polarization switching to domain-wall (DW)-mediated switching, accompanied by a coercive field increase of over 50%. Shannon entropy analysis demonstrates that this transition is driven by size-dependent polarization fluctuations that promote 180 degree DW nucleation - establishing a direct, quantitative link between local configurational disorder and macroscopic switching behavior. Additional analyses further suggest that the switching pathway is also influenced by the kinetic competition between DW nucleation/growth and annihilation. Furthermore, the switching pathway and hysteresis response are shown to depend critically on supercell geometry and the relative orientation of applied stress and electric field. These findings reveal that homogeneous and DW-mediated switching are distinct physical regimes in BaTiO3, and that atomistic simulations must account for system size to correctly capture the operative switching mechanism.