I run a Coralife Turbo Twist UV Sterilizer on my tank. Use to run it 24/7, but now I run it every now and then, or when I see ick, or the fish don't feel too well. It is great if you are just starting a tank and youa re adding new fish.
I would have to agree Aqua U.V.'s are the best but I'm not to fond of the wiper they get dirty and you wind up smudging the quartz sleeve. I'd say get an Aqua U.V. but without the wiper.
as far as size you'd have to match it with the pump that would be feeding it