At a panel not too long ago I was asked how I decided how long to make a story. My answer at that time, somewhat facetiously, was that a story was as long as it has to be and not a word more.
In the past year, in addition to working on some novels, I deliberately set a limit of no more than 7,500 words as the upper limit for any short story that I wrote, with a strong preference for keeping them around 5,000 words. Of the eight competed, I managed to write just three within those limits. Four went over the 7,500 word limit and, sadly, the final one is now threatening to become more than 20,000 words and become practically unsaleable.
So why couldn't I stick to my goal and make them all short? At what point did my stories become longer than planned and why? I honestly don't know the answer to that question, but I do know that there was no point at which I said "enough!" I do not pad the stories, adding unnecessary details and diversions that wander far from the plot or delve too deeply into descriptions, internal ramblings, or the price of tea in China. No, I set out with my plots sketched out, knew where the story line was going to end, and how I wanted to develop the idea(s) but ...
Here's a hypothetical: To get from dramatic point A to dramatic point C something had to happen - perhaps a new character needed to be introduced, a bit of backstory, a little foreshadowing, or a piece of exposition. The introduction of this material isn't really a choice but called for by the demands of the story as it strives for completion. Without that something being introduced, the story would fall apart or fail the reader in some way. You can't fight it, so you do what is needed and welcome the something into the flow and that gives rise to another scene or two, some more interactions, and, before you know it, the story has taken itself over the limit. Worse, when the final editing is completed, and when all the fluff and nonsense has been removed you discover that the story has resolved into the size it was meant to be.
So who decides on the length of the story being told? Did the plot force it to be a certain length? Were the characters' backgrounds and personalities the driving force? Was it the descriptive density of the setting? Perhaps it was the sheer number of interactions that had to take place? So many factors, so many choices and who is the writer to decide?
A story becomes as long as it needs to be.