What is the largest tank that you would consider using a naturalfiltration system on

I'm getting curious....having got the 15 gall nano tank up and running
and my main issue being algae but very little else (I come from a
freshwater tropical backgroud and algae is always an issue that needs to
be beaten) I'm starting to wonder about the high tech stuff....

From some reading I've done it should, in theory, be possible to set up
a larger tank using just sand and live rock as a filtration system in
the same way I have with my Nano tank - I hasten to add that the protein
skimmer has proved to be a necessity....from other reading I have seen
it suggested that using a conventional filter system promotes nitrates
which is far more of an issue in a marine tank than a freshwater -
afterall in a freshwater I can just throw in more and more plants to
swallow these up....

So, back to the question....what is the max size tank anyone would
suggest for a filtration system based on Live Rock and a good protein
skimmer?

TIA
Gill