The resources are mainly at Bored of Studies as well as the actual Board of studies.
Though as for the length of your essay, it really varies from person to person, as well as the size your handwriting, each word, etc. Because keep in mind, lol the booklets you use to write your essays in the HSC are mega huge, those booklets are usually 8 pages. I went through about 1 and a half booklets for each module in the recent hsc.
I wouldn't really recommend focusing on your length of the essay since you've still got a whole year, rather I would recommend you to piece together all the information you need for the essay, structure it, then do your S.E.X.Y. paragraphs, its surprising how easy Essays are by the time you actually get the hang of essays...so yeah xD You can't control your length of your essay until you gather everything, then summarise which and what you want in your exam, this is especially good for testing in Trial HSC.