It's quite simple. Simply click the "Edit this page" tab located at the top of the page or the other edit link across from headings on the right hand side of the page, and type away. See How to edit a page to learn about making links, using bold and italics, linking to images, and many other things...
Registered users can move a page; this moves the page content and edit history to a new title, and creates a redirecting page at the old title. This method is better than just copying the content by hand, as it preserves the article's history. Use the "Move this page" link. If you want to move a page, please click the "What links here" and fix the links to the page in question. See How to rename (move) a page.
Since one can link from page to page to page, then how long should the ideal Wikipedia article be? A good rule of thumb would be less than 5000 words, unless the subject really, really needs much exposition. However, for a subject that is that complex, one can link several shorter articles together, using a hub page to tie all articles together.
History of Foo
Physical Descript of Foo
Relationship with Bar
Modern Cultural Icons and Foo
If you write one long article, you will need new headlines anyway. If you write a long paragraph, then you need to add new linebreaks. The structure of Wikipedia is a web, instead of a text that you read linearly.
Search results give the size, for example "Knot theory (3220 bytes)". If search is disabled, copy the content of the edit box into an editor, and save as a text file, and check the properties of the file. Some variation is possible depending on whether a new line takes one or two bytes.
When the article size reaches 32 KB a warning is displayed at the top of the edit screen, as that size gives some browsers problems. It will likely give readers and editors problems, also.
How do I determine what other users have changed in an article?[ādihtan fruman]
Wikipedia's software can produce a list of all the changes between two versions of an article (either between two consecutive versions, or between an old and the current version), laid out in two-columns side by side with changes highlighted (here's an example. From the Recent Changes page you can click the "diff" link; from an article page itself click "Page history", then "cur" or "last" to see changes.
To see the differences between two arbitrary versions of an article, see Wikipedia:URLs.
First, you need the right to publish the picture under the GNU Free Documentation License. This means that either you created the picture and therefore own the copyright, or it is in the public domain. If the picture is located on a server you control, you can refer to that image from your wiki page by simply including its URL, like this:
and it will be included. (Note it will only be linked, not displayed.) If instead you want to upload a picture to wikipedia.org, you can use special:upload as a logged-in user and once it is uploaded, you can refer to it in your wiki pages as above, by including its file name :[[image:NameOfImage.png|Alternate Text]].