I would say 10,000 words or less for an original piece, the same as a non-novella/novel fanfic.
Since someone mentioned a rating of "M/MA", could you please clarify for folks if there is any difference there?
Perhaps it is similar to the difference between "R" and "NC-17"?
It is a similar distinction. (check out ->
http://www.fictionratings.com/guide.php for more info on fiction ratings!)
M - Mature - is listed as a 16+ rating and can contain non-explicit adult themes, hard/ course language, and references to violence. Its recommended for mature teens and adults.
MA - Mature Adult - is listed as 18+ and recommended only for the adult crowd.
One easy distinction between a T (13+) rating and a M or MA rating is the level of sexual content or sexual based swearing. For example, the MPAA will let the 'F-word' slide in a PG-13 movie once or twice so long as it is not used in a sexual context. The same goes for most swear words.
As for sexual content, if it goes past 'making-out', and hands/ other body parts go 'below the belt' so to speak, you most likely have an M or MA rated piece. Now the main difference between fiction and movie ratings in this case is that in a movie it is shown, while in fiction it is described. In a T rated piece you can still mention that a character has engaged in sexual activities, so long as you do not go into detail. Remember that is a 'mention', not an explanation of the deed, or all the 'dirty details'. So even though you have a T rated piece it doesn't mean you have to skip over the subject of sex altogether...however it needs to stay within the range of 'civil conversation', ie. would you be allowed to talk about it in a middle school, summer camp, or work setting?
For violence levels, generally there should be limited graphic violence (meaning: violence that is described or shown in detail), as well as limited to no blood and gore, in a T rated piece. Just because someone is killed, doesn't automatically mean your writing should get a M/MA rating, it depends how much detail you put into your descriptions. If you're writing out the details of someone being drawn and quartered (torn limb form limb), and are including things like bones snapping, and skin ripping and the like, you're most likely going to end up with a mature rating. If you leave out those details and only write: 'The condemned prisoner was torn limb from limb in the courtyard.' you'd still be safely within a T rating. If you have a Fantasy or Sci-Fi piece this can be harder to judge as its normal for a knight to go around killing dragons, or for a spacer to fight for their life against an alien race...the best advice for judging violence levels in that case would be to ask yourself (honestly) or a friend: Would this be too violent or scary to be in a PG-13 moive?
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(The Wiki Article on MPAA movie ratings ->
http://en.wikipedia.org/wiki/Motion_Picture_Association_of_America_film_rating_system)
R - Restricted - 17+ (18 in some states), someone 21 years or older is required to see the movie with you if you are underage, or do not have an ID to prove your age. "May contain very strong language or strong sexual emphasis, strong explicit nudity, strong violence and gore, or strong drug content."
NC-17 - No One Under 17 - Self explanatory, no one under the age of 17, even with a parent or guardian will be admitted. "May contain very strong sexual or offensive language, strong explicit nudity, very strong gore or disturbing violence, or graphic drug abuse."
