The longest story I've ever written is definitely the one I posted on this site. It's about 50 pages long when single spaced. (86 manuscript-style.) So far it's almost 23,500 words.
I still have a lot planned for it. Two parts that will both be longer than my completed Part One. (Part One had some limited events. But I didn't feel adding length or extra events would benefit it at all.)
The story itself has been in the works for almost two years. But I've stopped writing every story I've ever started because of depression kicking my ass. Once I picked the story back up, I busted out, going from 12 pages to 86 in a little less than two months!
(I've gotta get in that groove again for Part Two and Three.)
I can't write short stories. I was in a college course for short stories and the idea I had on the first day is now my completed "Part One." Most of the feedback from my professor came as compliments. (I wrote a psychological fantasy story he actually enjoyed, when he said not to do fantasy or horror in his class!) But the worst feedback he gave to me after class was that I "suck ass when it comes to writing short stories."
I'll always stick to longer stories when writing. (I've been world building for over seven years with a friend, and that world is where my stories take place. We flush entire lives out in our world. I don't think my brain can comprehend the creation of a "short" story anymore...)