Maybe the definitions have changed recently, but in all the time I've been reading fanfiction a "one shot" can be as long as you like. It must be posted in one shot, ie. no chapter breaks or waiting for the next part to be released. Some of the best fics I've ever read have been lengthy one shots. They have a different pacing to multi-part releases because the author sees the whole story before them, in its entirety, and can modify the beginning as they're writing the ending. Kinda like movie vs. TV series. This is how it's been classified for many years in other fandoms. Whilst Wikipedia is certainly not a definitive reference, seeing as they back up my position I'll quote them anyway: "Single-chapter stories of any length are usually referred to as one-shots." Yak.