This is never an easy question for writers who fall in love with their characters, who turn them into real people in character-driven novels. (Of course, in the plot-driven novel, the characters are sometimes little more than game pieces to be moved about at the author's whim.)
If you're writing the character-driven novel, the temptation will be to put in every single loving detail you've crafted. Before you do, though, ask yourself how important it is for your readers to know that the protagonist only uses peppermint-flavored toothpaste and won't abide spearmint, that he only shops on alternate Thursdays (unless it works into the plot), or that his aunt's second cousin twice removed is flying to Europe (unless the cousin is part of the plot).
The trick with writing, whether it be novels or short stories (but especially in short stories), is to know what to leave out. There's no easy way to know what is too much information, but here's a simple rule of thumb:
If it isn't moving the story forward, it can probably be cut.