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Silicon-on-insulator (SOI) optical waveguides with high electromagnetic field confinement suffer from sidewall roughness which is responsible for strong scattering effects. This letter reports a numerical investigation on the size influence of ultrasmall SOI waveguides on the propagation loss due to sidewall roughness. It is shown that for a size smaller than 260 × 260 nm the roughness-induced propagation loss decreases. As the optical mode confinement is reduced, a very low loss light coupling from and to a single-mode fiber can be achieved with propagation loss as low as 0.5 dB/cm for a 150 × 150 nm cross-sectional waveguide.