How do you measure a fan? I am trying to find a grill for these monsterous fans I have.

It is approximately 5 inches from left to right corner (upper corner to upper corner) and 5 inches from top to bottom. Approximately 1.5 inches thick. The writing on the back of the fan says "12V-(6-14V-) 6,5W 3350U/min" So I know it will run on 12 Volts and pull about 5 to 6 Watts but what is the 3350 U/min?

Thanks