Well this question comes up every now and again in different classrooms (though I swear each of my teachers has had different ideas about the length/amount to write), but the few generally agreed on were that:
1 or 2 Mark answers - are most likely going to be a one word answer; either 'list" or "identify" - and possibly one sentence.
3/4 mark answers - May need a definition (depending on subject) or just an explanation along with the given answer, and should be about the length of one paragraph.
5 Mark answers - should be either 2 short paragraphs or one long paragraph (and should more or less appear to take up half a page)
8 Marks - One page
10 Marks - A page and a half
12 Marks - two pages
15 marks - 2 and a half pages
Then you get the 20 and 25 response questions that, for some reason, I've heard:
* 3 - 5 pages
* Minimum of 8 pages
* Plus hearing about other people writing up to 12/13 pages
Which seriously conflict >.< But oh well. If someone on here knows and would like to, feel free to correct me (because its definitely something I'd like to know
) - But based on what I've heard from school (see above) I'm fairly confident about the first few, so, I guess your 6 mark question should be answered using about 2/3 of a page.