We present an experimental technique allowing size-based all-optical sorting of gold nanoparticles. The technique is based on the red-shift of plasmon resonance, due to retardation effects, with increasing particle size. As a result, smaller gold nanoparticles are influenced strongly by shorter wavelengths whereas larger gold nanoparticles are influenced more strongly by longer wavelengths. We utilise this retardation effect and realize sorting in a system of two counter-propagating evanescent waves, each at different wavelengths that selectively guide nanoparticles of different sizes in opposite directions. We validate this concept by demonstrating bidirectional sorting of gold nanoparticles of either 150 or 130 nm in diameter from those of 100 nm in diameter within a mixture.