Not sure myself. I would try contacting some filter manufacturers. Marineland has a toll free number and are very helpful but I don't think they make pond filters, everything else though.
Keep in mind that volume is secondary to the amount of fish.
There is tons and tons of information. You can start here . Also you can google biological pond filtration and mechanical pond filtration and you will get all the information you will ever need.
At the risk of becoming a broken record, try www.fishpondinfo.com for all your ponding needs. Robyn is my friend, and unlike so many webmasters out there today I personally KNOW she has real skill and experience when it comes to her ponds.
She has a newly setup forums and you can also email her any time...that is if you dont find everything you were looking for on her website.
If you are looking for a great website to find info this is one of the best ones that I have ever seen. Roark's Page
You need to sign up for his page but is well worth it. It totally blows away any and all.