Flashbacks come in all lengths, from many pages to a handful of lines. When you're deciding how much space to devote to a scene from the past, consider its importance. A flashback that reveals something unexpected may need more space than one that simply illustrates a reality about the character's past. Many failed flashbacks suffer from too much detail or too wide a scope. So, always make sure you include only what's necessary.
In F. Scott Fitzgerald's novel The Great Gastby, the flashback of Daisy's drunken episode the day before her wedding spans about a page. Fitzgerald focuses on the meaningful details: the letter (presumably from her true love, Gatsby) clutched in one hand that she gives up only when she "saw that it was coming to pieces like snow," the string of pearls valued at three hundred and fifty thousand dollars she insists should be given back to Tom because "Daisy's change' her mine," and the spirits of ammonia and ice on the forehead ministered by her bridesmaid to help her out of this episode. The scene reveals something important about Daisy's marriage to Tom and what that might mean for Gatsby, who is hoping to reconnect with her.
In Colum McCann's novel Let the Great World Spin, a brief flashback illustrates Corrigan's childhood understanding of his parents' relationship and his own youthful perception of his father:
Our mother found us one afternoon, dressed in his gray suits, the sleeves rolled up and the trousers held in place with elastic bands. We were marching around in his oversize brogues when she came and froze in the doorway, the room so quiet we could hear the radiator tick.
"Well," she said, as she knelt to the ground in front of us. Her face spread out in a grin that seemed to pain her. "Come here." She kissed us both on the cheek, tapped our bottoms. "Now run along." We slipped out of our father's old clothes, left them puddled on the floor.
Later that night we heard the clang of the coat hangers as she hung and rehung the suits.
In approximately one hundred and twenty words, McCann brings this moment to life with precise details: the grey suits, the trousers held in place with elastic bands, the clang of the coat hangers later that night. The mother's reaction—a pained grin, a tender gesture sending the boys on their way—is also quite revealing. It doesn't take a lot to make a moment vivid and meaningful, but it does take careful attention to detail.