I have come across author instructions for conferences that differentiate between full papers/short papers/poster papers by length only. Sometimes shorter paper types seem to undergo less scrutiny. Let's say I have made a significant finding that can be well expressed in 1 or 2 pages (shouldn't we all aim for being brief?), how would I decide what track to submit to? Why wouldn't I choose the poster/short paper track? Is there a difference in reputation between a short paper and a full paper? How would a reader be able to tell (other than by page count)? I've sometimes seen papers prefixed as "Short Paper: ...", but is this the norm in Computer Science? How do IEEE/ACM handle this?
Note that some "posters" are for work in progress, not completed works. I met a lot of students in ACM poster sessions who were "not quite done."
I would suggest submitting as a short paper if you have a significant result. The program committee might ask you to move it to a poster, however. But, for the initial submission, probably short paper is better.
Reputation likely isn't affected by the distinction between long and short papers, since some significant things can be expressed compactly. I don't think that would be the same for a poster, though not all conferences are equivalent.