A study on size-dependent subband structures of silicon nanowires (SiNWs) aligned along  direction, ranging from 0.77 to 2.69 nm in width, is performed by the first-principles calculation. Combined with a compact model adopting Landauer's formula, on-currents of ballistic SiNW field effect transistors (FETs) are estimated and assessment of size-dependent performance is conducted. Size-dependent injection velocity strongly depends on Fermi level measured from the conduction band edge when carriers are degenerate. It is also supposed that the Fermi level has a peak value at a certain wire width. Despite variation in the size-dependent Fermi level and injection velocity, large SiNW FETs show large on-currents owing to their larger gate capacitances resulting from longer periphery. The on-current in the case of a multichannel SiNW FET reveals that size-dependent subband structures of nanowires have a serious effect on performance. As the results, although the normalized on-current decreases with decrease in wire width in the assessed multichannel FETs because both the saturation velocities and the normalized capacitances decrease, there can be a maximum in the normalized on-current in a larger wire owing to the maximized Fermi level and injection velocity.