The most important thing in geo extended responses is that you're clear with the points that you are making. This means
don't repeat yourself just to make it a page longer, but use plenty of examples and statistics if you think it's going to be short.
Also, you
must make sure that you read the question carefully. If it asks you to analyse or evaluate the response is likely to be longer than something where you just have to describe or outline. It always depends on how confident you are with the topic and how much knowledge you have though.
It's said that the first twelve marks are easier to get than the last eight.
Length of the response depends on whether you want a band 5/6 mark or not. I would say to get 15+ you need at least 2.5 A4 pages, or more than half a booklet. That's if you write small and you're very concise. I honestly do not think you can answer an extended response well in one or two A4 pages. My half yearly average was about 3 A4 pages, to get 16+, for the trials I wrote 5 to 8 booklet pages for 17+. I think I got full marks, or close to, in the HSC and I wrote a full booklet for one question, a booklet and a page for another and 1.5 for the third. I have huge booklet writing though.
I personally preferred to write in essay style because it was good practice for other subjects. It also helped me to structure the essay, and my teacher found it really easy to read. Using subheadings is good when they are appropriate.
And I agree with lukebennett, try to leave two hours for the last section if you can, because it's more advantageous to do well in the essays than it is to do well in the other sections because 60% of the paper is the essays.
But also, as everyone said, don't go over the time limit! If you've spent 40 minutes on one question then you're cutting away time, and marks, from other questions. Keep your best for last to maximise time on the others, but don't sacrifice 5 marks because you were trying to write a conclusion for one of the responses.
The most important things for getting band 5/6 in geog [everyone wants that, don't they] are:
terminology,
definitions and
examples. Use these well and you can guarantee 19+. Work hard before the trials on writing a sophisticated and cohesive response and you could get 20.
Sorry for that essay.