How long should a funny story be?

It depends on what you want to do with it. If you're looking for a quick anecdote with a one-liner punchline, about 1 page worth of words should be your absolute maximum. If you want a short story, anything from 5 to 30 pages is common. There are stand-up comedians whose show consists of an hour of material tied to one main storyline. Generally speaking, I've found that the longer the story, the more depth it will need to have to keep readers invested, and to make them feel rewarded for their time investment. Longer stories will also need smaller jokes peppered throughout, even if you're leading to one major punchline, otherwise you'll lose your readers in the setup.