I know I shouldn’t bump this thread or anything, but…
Re “knew”: are you talking about rational people? Nowadays people “know” the earth isn’t flat… except for the ones who insist that it is. So depending on how rational your characters are, the story could be set earlier or later.
And re. bumping, please don’t do it, specially after such a short time. Give people time to answer or all you’ll manage to do is irritate them into silence.
Alright, I won’t do it.
And as for people being rational: The story is set in an “Alternate Timeline” Earth, where the only difference is that this landmass exists. So their rationality isn’t on the line here; mine is. 
Sorry for double posting, but it seems more appropriate to post this here than in a new thread.
If I choose to upload a short story or a chapter-by-chapter full-length story on a forum to get reviews, will my decision raise any problems in later publishing that story, either as a part of a short story collection or as a full novel? I know many authors did this in the past with magazines, but the 'net may be a different story.
If you do it here it becomes the intellectual property of the forum’s owners. So, don’t.
You can create a webpage to publish your work, some people have done that and later published it in hardcopy.
What if I post the actual updates on my own website, but I tell people about it on a certain forum?
Not sure if it’s relevant, but as far as size goes, here is a list of countries sorted by square kilometers.
It turn out Afghanistan is EXACTLY 250,000 square miles, if that data chart and this conversion calculator are accurate.
So that’s pretty small for a lost continent. More than twice the size of New Zealand, but less than a tenth the size of Australia.
I see what you did there, but I still think it should be a word.
Whoops. A lot. Sorry for that. English is not my native language, and I did not have a spell checker on the computer I typed that on. (Internet Explorer would not spell check for whatever reason). And indeed, it SHOULD be a word, like whatever, indeed, and unsportsmanlike. It has entered common usage, and in a few years, alot may be considered correct.
As for 250,000 square miles: That area is also slightly larger than Madagascar. And it’s not a lost continent, just an unexplored island. It just so happens that an island of that size can support quite a bit of life.
EDIT: And it seems “alright” is the proper spelling, while “allright” is wrong. Or did I make another mistake?
Damn English spellings… This is why I proofread AND have other people proofread some more.