They are pretty easy to size. Alot of descriptions give ratings as GPM @ say 30,000 uws/cm^2. Multiply the gpm by 60 to get GPH.
Then, divide the GPH until you have the desired uws/cm^2 rating for whatever you are trying to kill. As long as you are running your tank volume through the UV at least 1 x an hour, you will be fine.
Here is an example:
We can use Tanya's 100gal water volume. Depending on how much and what you want to kill, your uws/cm^2 will be the starting point. Lets say you want to kill anything that could possibly hurt your fish and you want 400,000 uws/cm^2.
For this example we can look at using a 40w emporer UV that is rated at 26.2gpm@30,000uws/cm^2. First convert to gph so 26.2gpm x 60 = 1572gph. 30,000uws/cm^2 is not enough so divide the 1572 gph x 2 and you get 786gph. At 786 GPH you are killing with 60,000 uws/cm^2. Still not there, divide by 2 again and at 393gph you have 120,000 uws/cm^2. Again and you have 196.5GPH @ 240,000 uws/cm^2. Divide it out one more time and you get a kill rate of 98.25GPH@480,000 uws/cm^2. This is just under 1 turn over an hour and you can kill just about anything that hits the water. But...this is when the bulb is brand new
. In 6 months, your only going to have 288,000 uws/cm^2 if you look at loosing 40% efficiency over the period. Take this into account when sizing your UV's. If I had 100g tank, and I wanted to kill at 400,00, I would go slightly bigger than 40w but it will work for 6 months to kill anything 288,000 or less.
Whenever you size, size it off of the lamp output rating after 6 months of use or a year of use or whatever the manufacturer provides. If they don't provide that data, then take 40% off of the rating they give you as new and use that. Hope this helps!