There's no firm and fast rules for how much filtration you need for a specific tank. Bigger is better (within reason, obviously) in this case.
When it comes to figuring out what filter suits your tank, go by the manufacturers information, and presume that they are "overestimating", so if they say that a filter is suitable for 25-50 gal, and you have a tank of (say) 38 gal, it's probably on the smaller end of what you want, and the next one up, say suitable for 40-80 gal would probably be better for you.
The other side of things is that it all depends on the feeding and stocking levels, and by that not only how many fish but also how big they are.
The best way to know that you have sufficient filtration, it's best to check your water for nitrate (and ammonia and nitrite of course) before you change the water. If it's higher than 25 ppm (or so), then you should either do bigger/more frequent water changes, or get a better filter, feed less or reduce the fish.
Also bear in mind that filters that pump a lot of water aren't necessarily better than ones that pump less water. The bacteria living in your filter media will need time to process the dirt, and they do that best if the water is flowing past at the right speed. So the right combination of filter media, canister size and pump size is what we want. I'm sure that Eheim, Fluval and other guys have some sort of formula to work this out, but they aren't going to tell us what it is...
Faster water-flow will of course increase oxygenation of the water, which is a good thing.
Finally, there are SOME species of fish that don't like too much water-movement. Most species of fish don't mind a bit of movement, but for instance Betta Splendens (Siamese fighter) can't build a bubble nest if there's too much turbulence in the tank. Most cichlids are fine however.