Ahh, nifty. In one study I read (http://www.scielo.br/scielo.php?pid=S1517-86922003000300005&script=sci_arttext&tlng=en) they are defined as having 6 to 12 carbons, and that seems to be the general rule. However, nearly every study on MCTs uses the standard pharmaceutical MCT oil which has very little C6 or C12, so I'm not all that sure where things stand, then. I doubt that C12 is a magic cut-off point - probably between C12 and C14 things get blurry and multiple metabolic pathways come into play.
Moral of the story? I'll keep taking some coconut oil with my flushes, but never too much.