calculating ring buffer size

Hi , While going through a few usb drivers code , i observed that the drivers maintain a ring buffer which stores data coming from usb device in interrupt context . Also these ring buffers is a power of 2 size i am looking for reasons how to compute the the max size of these ring buffers , so that no overflow situation happens . What factors should always be accounted while computing the max size of these ring buffers . Regards Amit Nagal