I usually write at least 3-4 solid paragraphs but then that's normally what I give in a tag and I feel my writing sample should reflect me. Also when apps call for you to write one just for the game it's often so one can get a feel for the characters voice and personally I don't see how one can do that with only one or two paragraphs. If a sample is just to see if one has a grasp of proper grammar and spelling, well the app reflects that pretty well. And I realize this answers more of how short is too short. As for how long is too long? I feel asking for more than 4 paragraphs in a writing sample is too much. Or if you wanna go by word count 250-300 words seems fair.
If a game doesn't call for a specific sample just for the game I keep log and narrative samples in my cdj that I link to so the mods can get a sense of my writing style. This is usually an easier route to go if you're not wanting character specific samples.