I don’t know if you’re ever going to find an absolute solution to this problem, because what we have to say varies from time to time. Even on Twitter, you can still write a 5,000 word piece, you would just be breaking it out tweet-by-tweet, instead of in one long thing. But you’re right— what you are presented with does influence, to some degree or another, how much you write.
But what if you thought about writing in a different way? If you were having a one on one conversation with someone over coffee or a beer, or if you were telling a story over a campfire, or if you were catching up with a distant friend over the telephone, would what you have to say be limited by the environment your words would be shooting out into? Would they be influenced by who was listening, by the amount of time you had to talk, by whether or not it was an intimate setting or a large cavernous auditorium?
What is writing anyway? Often, it’s just a written expression of an idea, a thought, something that happened, something you felt, something someone else felt, an explanation, a discussion, a story about something. But it comes from, I think, an oral tradition. I would imagine man/woman was speaking long before he/she was writing (although I’m obviously not an expert on this and just speculating). So these white spaces, essentially, they’re just there to get out something that might have, a million years ago, just been spoken aloud (ever wonder why writing from the 1700s sounds so weird? They likely spoke like that).
So, if you think about your writing more in that way, maybe the canvas you’re painting on— Medium, Tumblr, Twitter, etc. — won’t make as much of a difference. Look at it less about filling up the space that is provided, and more about this question of— what is it that I want to say, literally? I imagine that you will find the medium something of an afterthought at that point.
One very practical way of implementing this is to actually just speak your piece into existence. And what I mean by that is to sit there and maybe jot down a few questions for yourself that you’re looking to answer— you’re probably thinking about those questions anyway, because that’s what is giving you impetus to write in the first place— and then actually open up the voice recorder on your phone and talk into it like you’d be having a conversation with someone. You’re kind of interviewing yourself.
Then, go back and maybe transcribe a bit of that, or pull out the pieces that you find particularly engaging, and let that be what you write. In that way, the little box that you’re typing into, the white space, the thing that inspires you to write something long or something short or something neither long or short, won’t make nearly as much of a difference. Again, think of it as something spoken— a conversation with a friend— and filling the space will matter less and less.
Writing is, at the end of the day, just a vessel for delivering something that can be expressed in other ways.