Ok, first of all, get your minds out of the gutter.
Second of all, I think the answer to "are longer fanfictions better?" is complicated.
But before we get into my thoughts on the matter, let's define what I mean by a "long" fic. For me, anything under 20k words is short. Between 20k and 40k lies a gray area which I wouldn't necessarily call long or short, but if pushed I would probably lump into short because I tend to finish fics of that length in one sitting. From 40k to 90k, I would consider a normal length because that's usually the range in which published novels usually fall. Over 90k to about 200k words is what I would categorize as long, and anything over 200k would be super long.
Sometimes, I love a super long fanfic. There's almost nothing better than finding a great 250k word fic for your favorite pairing with an awesome plot and amazing storytelling. Being able to immerse yourself for a couple weeks (or a few days if you're a crazy fast reader like me) in one story and not having to reintroduce yourself to new characters every few thousand words is great.
I tend to get much more emotionally invested in a longer story than in a short one. It's rare that a 20k word or fewer fic will make me cry—and I really do take into account how much I cried while reading when deciding if a fic was good or not. The author is able to spend more time exploring a character's motivations, backstory and feelings in a longer fic, which makes a reader feel more connected to them.
But the problem with fics that are over 200k words is that they can really drag on. While half of the time, I love some inner monologue, lots of scene description and crazy detailed interactions, the other half of the time, I get bored. There is a fine balance between just enough details and too many.
Unless a story has a short timeline in which everything is happening very fast with no breaks, I don't need to read about every day of the characters' lives. If nothing important is happening between Monday and Thursday, then I don't want to read about the main character just hanging out at home on Tuesday and Wednesday. Time skips are a beautiful thing sometimes.
I also don't need a character's breakfast or outfit described in 1,000 words of detail unless it's integral to the story. And even then, 200 words of oatmeal is more than sufficient.
Keeping track of continuity is also a lot harder for an author of a super long fic. I typically can finish a super long fic in three sittings or less, depending on how much free time I have at that time, and am much more likely to catch a continuity error than if I was taking weeks to finish reading. So mistakes tend to stick out to me and knock my enjoyment of the fic slightly down.
I have a similar problem with super long fics and repetition. I read a Game of Thrones fanfic recently (I won't drop names) that I had high hopes for, and the story was good, but the 150k words could have been knocked down to 75k if all the repetition of characters' thoughts and motivations was cut down. I found myself getting bored of the story about 2/3 of the way through, but I wanted to see how it ended and I was already so far in to the long fic, so I stuck it out, though I was relieved by the time it was over.
But if a fic is super long and has a good story and great characterization and I'm invested in it, I'm more likely to ignore things like typos and small mistakes. I have only read one fic that was over 450k words (and lord help me, it's a WIP) and the story was so good—just enough canon to not be considered a full AU, but with enough changes and original characters to make it a masterpiece in its own right—that I almost didn't care that there were fairly frequent typos. I barely cared the couple times the author called a side or background character the wrong name.
(Super long WIPs are a whole other beast and I will write about the pros and cons of those in another post, I promise.)
So, despite the issues that can come with long and super long fics, I love them very much, and most of my favorite fics fall into one of those lengths. But I also have times when I don't really want to invest the kind of time and commitment that a 200k word fanfic takes, and a short fic is what I want.
I usually lean toward short fics when I want something cute and happy or fun and fluffy because as I said above, it's hard to cram enough story into less than 20k to make it an angsty and emotional adventure (hard, but not impossible). When I first start reading for a fandom or a pairing, I also tend to start with short fics to get myself used to the new type of fic. Jumping into a new fandom or pairing with a 200k fic is not something I typically do, unless it's a rare pairing and that's all that's available.
My only real problem with short fics is that they don't tend to leave an impression on me after I finish them. I read the 15k words, close out the fic and move onto the next one with little feeling other than, "Hm, that was good," or "I'm glad I only wasted an hour on that."
I mainly avoid fics that aren't at least 1,500 words unless they are the only ones left. This is only because it doesn't usually seem worth it to me to spend time downloading the epub and sorting it into my ebook library to only spend five minutes reading the one shot.
My recommendation list, which is just my bookmarks page on AO3, has some short fics on it—mostly from when I only read 1D fanfics—but they are easily overshadowed by the ones 40k and up.
While there are certain times when I'd prefer to read a short fic over a super long one—and vice versa—I wouldn't say I prefer one over the other. If I was forced to pick a favorite fanfic length, I would probably pick the novel-length ones because there's enough room for plot and character development, world building and scene setting, without the added space for tedious descriptors and unnecessary scenes. There's a reason publishers prefer novels of that length range.
What about you? Do you prefer long or short fics? Are there lengths of fanfics you avoid? What's the longest fanfic you've ever read?