These can range from self-contained stories to prototypes to hentai.
Depending on the title, these can have only a few pages, or be 100+ pages long. Do not let this scare you. Treat it like any other synopsis and you will not have a problem.
As most one-shots are usually fairly short, you will not be scrutinized for not reaching the average word count (that is usually about 150 words). Try, however, to get it to at least half of that.
It is preferable to read the entire story before you start writing. Some things that could be used in your synopsis may not be revealed until later on (e.g. full names and school grades).
In some cases, no names may be given for the main characters. In that case, you may use words such as "a young businessman", "a high school student", "a private tutor" etc. all depending on who the main characters are.
The majority of readers are not interested in the story. Instead of solely writing what the story is about, try to also leave off with something that will get the reader curious—a sort of a teaser.