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Lateral size effects on surface-roughness-limited mobility in silicon-nanowire FETs are analyzed by means of a full- quantum 3-D self-consistent simulation. A statistical analysis is carried out by considering different realizations of the potential roughness at the Si-SiO2 interfaces. Nanowires with lateral section varying from 3 times 3 to 7 times 7 nm2 are considered. Effective mobility is computed by evaluating the electron density in a reduced channel region to eliminate parasitic effects from contacts. It is found that transport in wires with the smallest section is dominated by scattering due to potential fluctuations, resulting in a larger standard deviation of the effective mobility, whereas it is dominated by transverse-mode coupling in wires with larger section, resulting in a stronger influence of surface roughness at high gate voltages.