The stories for last year's SS are on the SS site, y'know. You could peek at them and see the length!
(Which seems to run from "a couple of pages" to "quite long" so I dunno that there really is even a trend.)
ETA: And I have a story idea now. A beginning-to-end, know-how-many-scenes, know-what-the-resolution-is-and-how-to-get-there story idea. Which ironically fits almost all of the parameters of the plot-with-het that I was worried about before, including the same characters, but is a completely different story, and doesn't require actual hetsmut.