This is probably the section which students consistently give much less time to than it needs. A clear and concise introduction, which sets out what you plan to do, is invaluable for an examiner.
Consider how valuable introductions (and abstracts) are for you when you are reading journal articles. They should tell you enough to allow you to assess whether it is worth continuing. So should the introduction of your dissertation.
The precise length of an introduction will vary from topic to topic, but at most it should not exceed 1000 words. In fact, anything much more than 500 words is likely to be too long – it stops you from spending those words on the main piece and it is likely that a long introduction contains a lot of unnecessary information.
An introduction should:
Set out the research question you are attempting to address
Briefly mention why it is of interest
Briefly explain what you are going to do (method, sample)
Allude to your conclusion (optional)
‘Signpost’ the rest of the dissertation. (e.g. ‘The paper proceeds as follows. Section 2 introduces the literature. Section 3 discusses the model… etc.’)
It should be stressed that these are suggestions. But none of the above should be detrimental.
There are many good examples of how to write an effective introduction in published academic journals. One seminal paper, by Orley Ashenfelter and Alan Kruger in the 1994 American Economic Review which looks at the returns to schooling for twins, is just one example.