You are limited to what they make... So if your tank volume + sump volume is 300 gal... you are looking for a 30W or larger UV light...
So I would suggest the Danner 40W... or Turbo Twist 36W...
I've used many models of those brands and am confident in them... I'm also quite well read on UV lights and I am not aware of any brand I would steer you away from. Just avoid the "foreign knock offs"...
You can shop around, you may find a 30W UV... I have a 13W which is an odd size... I have no clue who the manufacturer is, but it works great...