I just did one of those a few months ago. The content should really depend on if you're doing it to put out before the main project, or during it. Know that there is certain ground to cover in a short span of time. I'll use my teaser comic as an example, which you can see here: https://tapas.io/episode/670545
1st is setting, then introducing your characters. In mine, having the readers know their names and their general personalities was a priority for page 1. The second thing I tried to fit onto the first page was an idea of what the short story was about.
Once you've established those general things, pages 2-5 are room to show the reader what to expect. A little bit of world building and information of what kind of story you're writing. In my case, I was showing off ancient ruins, shadow monsters, and magic. Then of course page 6 was to wrap up the story and show some lesson has been learned.
Doing anything shorter than 6 pages would be incredibly hard for me, because lord knows 6 pages was a challenge. When I started, I just wrote up a small, contained story. It would have been about 10 pages at first, then I looked for every non-essential thing I could take out.
It's best to think of a small arching story that doesn't need a lot of explaining to get across to people. Show more, tell less! Use the format of comics to give as much information as you can, visually, in as few panels as you can.
Best of luck! I'd love to see what you come up with!