Most large earthquakes occur on several faults, and accordingly it is important to the prediction of earthquake size to know whether several adjacent faults rupture simultaneously and how slips are distributed on the simultaneously ruptured faults. In this article, I investigate slip-length scaling law, simulating spontaneous rupture processes on multisegmented strike-slip faults in a 3D half-space. Because of fault interaction, the amount of slip caused by simultaneous ruptures on two or more segments is larger than that of a rupture on a single segment. The fault interaction decreases with the distance between segments, thus the amount of slip attains a constant value when more than two segments rupture. The decrease of fault interaction also causes the small rate of slip increase for simultaneous ruptures on long segments. The numerical results physically support the empirical scaling law, which is characterized by a strong increase of slip with length and a tendency for slip to saturate for very long faults.