Uv depends more on flowrate than size of tank....The higher the flow the less contact time the water has with the uv light i know that some people have 15watters and bigger on 55s because they have alot of flow.....most likely you will be fine if you get an 8.....
Do heavily consider this purchase though...They are expensive and often do not contribute to the health of the tank...some say they hurt it.
I have a 15 on one tank with about 600gph running through it...I only turn it on when I put in new fish or inverts..