The amplitude should be proportional to the width. In single slit diffraction cslvulation, the resultant amplitude is goind by dividing the slit width into lafge no of eaual segments. For each segment, the amplitude is raken proportionally equal and a constant phase difference is taken as existing between adjacent segments. The resultant amplitude is found by superposition at the point of consideration. So amplitude should be proportional to slit width. The intesity is proportional to square of slit width, as intensity is proportional to square of amplitude.