"I understand the differenc from 1100mAh VS 1500mAh but whats the difference in size do? "

The mAh rating of the battery is the capacity, or size of the battery. The smaller the battery, the shorter the run time. A smaller Mah rating also generally means more voltage drop under load, but possibly not in this case if you went from AA to sub C, but without knowing the impedance of the cells, I can't really say. Using a 1100mAh instead of the 1500mAh should be fine, but you will have slightly less run time.