Fallacy?

"and after 3 centuries know that largest country in the world is china" I don't get this part... seems like something is wrong grammatically and I can't follow the meaning.

[To Bo Bennett, PhD]

I mean that when we see that largest country in the world is russia and after 3 centuries we see that largest country in the world is china and therefore we shouldn't assume that when people at 21st century refer to largest country as china when they use" I came from largest country in the world.

Russia is largest by geography and China is largest by population. Perhaps you are referring to the problem of equivocation ?

No I am referring to russia by how big it is and china by how big it is

Sorry, I am still really lost on this one. Perhaps someone else gets what you are asking.

I’m confused over the question and in particular the “living in the 21st century” reference.

Are you suggesting that a person from a different century might think otherwise because of what was known (or will be known) at that time?

Are you suggesting China is, in fact “larger” than Russia?  If so, what is your understanding of “large”?

I am telling that you are a 21st century person and you know russia is largest by area and after 3 centuries people of that time know that largest country in the world is china and not russia because china conquered lands of russia and became largest country in the world after 3 centuries and there is no reason to think that people of 21st century thought that largest country in the world is china because 24th century people reffer to it as china.

I'm assuming the title of your original post ("Fallacy?") is a way of asking if there are any logical fallacies in the 49-word sentence presented.  To more easily identify the presence or absence of logical fallacies, it generally helps to clean the discussion up by putting it into a standard logical form.  It also helps when one makes assumptions and definitions explicit so that no one gets misled by ambiguous terms or ones with two or more legitimate meanings.  (Failing to be explicit about definitions or assumptions can lead to the logical fallacy of equivocation – and that's one of the fallacies for which I see your initial post providing a good foundation.)

Putting the statement into logical form, help show links between elements of the argument – or help show how some points are irrelevant or add little to reaching a conclusion.  (Failing to link elements together or heading down irrelevant paths can lead to the logical fallacy of non sequitur – and that's one other fallacy for which I see a good foundation in your statement.)

Putting the statement into a conventional logical form helps identify the argument and the conclusion ... and neither is clear to me right now, so what follows may not be what you intended.  In a conventional logical form, my understanding takes me to something like:

Premise 1: A person living in 21st century believes that largest country in the world is Russia.

Premise 2: After 3 centuries, people will come to know that largest country in the world is China.

Therefore, 

Conclusion: "when people live in 21st century don't mean that largest country in the world is China."

Assuming we can agree on the meaning of "largest" (area?, population?, border length?, ...), it should be easy to tell if Russia is the largest country today.  However, that's not what Premise 1 discusses ... that premise discusses what "a person" (one specific individual?, which individual?, ... people in general?, ...) believe about the largest country ... I know of no evidence to demonstrate whether premise 1 (about "believing") is true or not.

Premise 2 makes a prediction ... "...after 3 centuries (someone will) know..." that China is at that point in time the largest country.  Since it is merely a prediction, it's not possible to demonstrate whether this premise is true or false (i.e., whether China will actually be the largest country by that time and whether someone will know that to be true) so the potential for a non sequitur becomes very large here.  Beyond this point, premise 2 actually speaks about what "A person living in 21st century" will know "after 3 centuries".  Barring any surprising advances in longevity, there's a huge non sequitur in considering what a person alive today will know 3 centuries from now!

There doesn't seem to be any link between what people believe today (premise 1), what people alive today will know in 3 centuries (premise 2) and whether people living today means that China is or isn't the largest country – I would assume that factors like population, area, etc. would be better determiners of China's size status than "people liv(ing) in 21st century".  It has non sequitur written all over it!

The absence of any explanation of what constitutes "largest" for countries lets participants interpret "large" in a variety of ways, leading to differing interpretations leading from different definitions – with equivocation  written all over it!

Speculating on what our contemporaries might know 3 centuries from now (assuming any of those of us live now will know anything at that point), brings us back to non sequitur because of the introduction of irrelevant material.

So, if your post was to ask if any logical fallacies exist in the sentence, I see at least 2 ... and perhaps more if I understood the argument better.