Help!! I am trying to write the intro chapter of my thesis- normally i would leave this till last but i have been advised of the importance of getting this chapter perfect right at the outset as it really sets out in your mind and on paper what you are going to do.

I am struggling with not only the structure of this first chapter- but how long should it be???

I think it can depend on what field you are in, but I would always advise leaving the introduction chapter til last. I am in the sciences, so it may be different in other disciplines, but I wrote all the results chapters and the conclusions first. Then it was easy to see what 'story' I was trying to create, so I could then go back and write my Introduction, as I knew exactly what message it needed to get across and which areas of the literature it needed to focus on in order to strongly relate it to my results and conclusions.
You could always write a rough outline of the Intro chapter before you start anything else, making note of important papers to include etc., just so that you have something down on paper. It should be a scene setter, so for me, I needed to include: background of the wider field (i.e. where did it start - my field is pharmacology so I started with the ancient Egyptians!), brief story of how it developed to present day, then the background of my specific field, important current/recent papers, then where my research fits, and where there may be gaps in the literature that my thesis will then go on to fill.
As a guide, my intro was about 8/9000 words out of a total of 55000.