The amplitude should be proportional to the width.
In single slit diffraction cslvulationcalculations, the resultant amplitude is goindobtained by dividing the slit width into lafge noa large number of eaualequal segments. For For each segment, the amplitude is rakentaken 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 The intensity is proportional to the square of slit width, as intensity is proportional to the square of the amplitude.