With all technology discovered prior to this question, on today's planet, there are still some things we have not observed - which may be used to justify introducing new life forms, right here at home, in a story.
There are a LOT of unexplored places on Earth
- We've explored less than five percent of the ocean
- We see evidence of potentially new organisms but can't always locate them
- Plenty of undiscovered species do exist
- Many rainforests have areas that are either uninhabited or that modern scientists haven't been through, ruling out discovery in the "traditional" sense but allowing large creatures to go unnoticed by science
To be clear
I am defining "undiscovered species" as:
- Somewhat unique. Cannot be a subspecies or a division of a well-known species into groups that are somewhat similar to each other.
- Unbeknownst to or denied by modern scientists. Can be seen by isolated or indigenous societies.
- Lacking observed evidence of existence to the point that any actual evidence found is explained by some other phenomena. Like if the bloop was an animal and we brushed it off as ice moving, or like if this thing wasn't actually a known animal, or if this thing wasn't a sleeper shark.
So the question is
Given the above information, how large can each of the following organisms plausibly be on Earth in 2016 while remaining undiscovered with current technology?
- Plant, fungus, or similar species
- Land animal
- Aquatic animal
*I will not accept "it will probably be..." I would like calculations or references as opposed to speculation, please.
Edit: In response to all answers - I ask "how large can they be" not "what is a size they could be". I do apprciate your answers but every one seems to say "this is a large thing we've seen so an organism could be at least this large". What no answer does say, as the question asks, is "based on the ways we observe the world, an organism could be at most this large".