I would say length between 3-4 paragraphs is good. As already stated by others. But it should be remembered that it isn't length that a lot of people might necessary be looking for, but rather quality compared to quantity. At least that's what I look for as a mod/admin to games. Although in actually tagging, the same amount of effort ought be given by all writing parties. I've seen that lacking a lot in some places. :/
As for the question too long? Well I'm one of those writers that can really knock out a lot of words with an array of my characters, but for samples 5-6 paragraphs would be a lot. You just want a short scene. Nothing for anyone to get lost in. Or my 2cents on it anyway. And, Links to various samples/examples is always nice. Get a well rounded idea of the players style.