They don't have a "limit;" they just split to multiple pages at somewhere between about 700 and 900 words. It's... weird, reading what I think of as "ficlets" split into two sections (oh, and you don't get any say about where the split happens; it can be in the middle of a sentence.)

They're obviously not collecting novels; they're expecting people to post single-paragraph squee fanfic. I've seen "fanfic" there that looks like a summary: Character X and Y are marrying. Z is unhappy, and decides to break up the wedding. Character X fortunately has kept his Magic McGuffin, and defeats Z easily. Everyone cheers. X and Y kiss.

Add three-hundred-word description of how beautiful Character Y's dress is, and you're all set.