Sample answers may be used by students as guidance when completing past exam papers as they provide insight into ideas relevant to a particular HSC exam question, allowing students to expand their thinking and effectively incorporate more/more diverse ideas into their responses.
Sample answers may not necessarily be full mark responses and should probably not be assumed to be full mark responses, although they seemingly appear to have particular standards in terms of quality, i.e. they seem to lean more towards the good quality end of the spectrum.
Regarding the length of the sample answers, it should probably not be considered as a standard for the length of your own responses. Instead, you can seek guidance through sample answers as mentioned, but the length of your own answers will depend on the question itself in addition to factors such as mark allocation and the spaces provided.
I hope this helps!