i don't find that i remember long fics more. if i remember a long story it's usually a sharp moment of impact that i remember, and i remember those from very short stories, even scene-length vignettes (although i don't really like one-scene stories much), too.

it's probably a difference in brain chemistry or something. my attention span is short in everything, not just stories. although i remember when i was a child reading novels all day, they have to be quite good, and it's easier to find high quality science fiction than high quality fan fiction. especially if your parents have a large private library of it... .