We show the importance of finite particle size in microfluidic asymmetric continuous-flow diffusion arrays, specifically the critical nature of the particle size with respect to the barrier gaps. We show that particles much smaller than the barrier gap follow individual field lines through narrow gaps and are poorly fractionated. In contrast, particles comparable to the gap size lose memory of their incoming field line and can be fractionated with high resolution. We demonstrate this effect using a new technological approach to create very straight and narrow injection bands in such arrays, and completely resolve bands of DNA of lengths 48,500 and 16,7000 base pairs.