I ask the question, not as a subjective "well, Dale, what do ya reckon?" but as a perfectly factual question about tree growth here on earth provided a finite set of physical (natural, real) conditions that are all, perfectly, verifiable.
Criteria or conditions that come to mind for "biggest possible tree on earth" (sorry for the trite superlatives, I put it in quotes facetiously actually) are
- Nutrient-rich soil
- Lack of outside interference that stunts the tree (harsh winters, humans, etc)
- Weather conditions
I listed gravity because if for example, gravity were twice as intense as it is right now, tree growth would be affected in much the same way that all kinds of animals and insects would be affected. In the past, as yet another example of the importance of gravity, scientists used to think that dinosaurs couldn't have been as big as they are presently held to be because the gravity of the earth would have made it so their bones could not support their own weight. Such reasoning was meant to explain the then-accepted expanding-world theory, in which scientists hypothesized that the earth, therefore, must have been smaller, which seemed to reconcile evidence of massive dinosaurs and moving continents. Of course, the theory was later falsified by the theory of plate tectonics, but that's beside the point.
When the dinosaurs roamed the earth, forests could expand as much as natural conditions permitted and, again to my example, if gravity were less than it is presently by a factor of however much, one should think that vegetation would flourish accordingly. Additionally, there were no human industries or activities and hence no pollution. Even today, trees that are left uncut as monuments of the past, expand to utterly unbelievable girths, and can sometimes reach mind-boggling heights.
All that said, a tree as imagined in movies like Avatar would seem ludicrously impossible here on earth, which is where one starts to wonder: what are the natural limits of trees in the best possible environment. Perhaps with something like nitrogen or say a kind of steroid-equivalent for plants, trees could attain far greater heights?
Of course, I emphasize the absolute best conditions or environment because that is the means by which the question remains answerable and yet adapted to personal curiosity. If yet another example should help, consider a tree that had all the water it needed (but not too much, i.e. a Lebanese cedar tree versus a mangrove tree), all the ecological conditions necessary to reach its maximum growth, (endless nutrient-rich soil, insects that didn't kill it, no diseases, no other trees whose roots poison it; no trees that grow over it and suffocate it--yes, some trees suffocate other trees), the best possible conditions of gravity, etc. Additionally, one can't forget the genus of the tree in question, since for example a dwarf willow is considerably different (and smaller) from say a giant sequoia: Clearly you would agree, I think, genius is a serious criterion if to answer the question.
On a side note: I admit that gravity might be a hard condition to humor, but in as much as less gravity would allow for bigger trees, it should also seem a crucial criterion in answering the question. That said, I recognize that I'm only the questioner and not the responder, so take it away. :)
Nearly last but not least: While it may seem foolish to leave out the fact of evolution as a primary criterion, the problem with including it as one--at least to my mind--is that one could misuse it to create false or unsupported assumptions. Consider the following truism and its conclusion: If you follow any evolutionary timeline of any given plant (say a fern plant), you see that various subclasses appeared (evolved) and disappeared in different geologic time periods of that plant; therefore, a subclass of giant sequoia could appear (evolve) to ten times the size of their present ancestors. As you'll no doubt agree, such would be far too easy. Hence, while evolution is no doubt crucial point to consider, hopefully, it'll be used advisedly.
One final point: If you think the question can't be answered for some other reason, please share why you think such so that I can edit the question. In other words: please show how it doesn't meet some standard versus just citing the standard. It might be clear to you why it isn't right, but "down vote" doesn't always tell the questioner why their question isn't right for the community.
Thanks very much.