The length of introductions vary depending on your disciplinary area and the nature of your project so before writing one, it may be wise to discuss length and expectations of content with your supervisor. It is also worth taking a look at a range of past theses in your area to see what is expected in your field.
The introduction to the whole thesis can make up roughly 10 per cent of the total word count. So if you are doing a PhD of 80,000 - 100,000 words, you may have a 8,000 - 10,000 word introduction. And if you are writing a Masters thesis of 15,000 - 20,000 words, your introduction could be 1,500 - 2,000 words long. Exegeses tend to be anywhere between seven to 12 pages in length (1.5 cm spacing) and include images and/or diagrams.
Theses that generally have longer introductions do so because they incorporate more substantial background information and/or an extended literature review section.
Theses that have shorter introductions, say eight to ten pages (1.5 spacing) or about 4000 words for a PhD, do so in order to give a broad overview of the research project which is typically followed by a longer background or literature review chapter.
Regardless of the type of project you are doing or thesis you are writing, the introduction to the thesis or exegesis should give a broad overview of the project, as discussed in the page on introductions.
Individual chapters should also have introductions. An introduction to a chapter that is 10,000 words long could be about two to three pages long, whereas in shorter theses the chapter introductions may only be a page or two, or even just a paragraph or two. The length varies depending on how much you need to establish for your reader to understand how the chapter contributes to your research and argument.
One supervisor rejected his PhD student's introduction of 30 plus pages, saying an introduction should be about 'eight pages and should cover context, issues and method in that order.' So the student re-wrote to those instructions. He initially established the context of his research by providing dense background information; he then identified the key issues in the process of defining his research objectives; finally he provided a conventional chapter-by-chapter description of the thesis procedure.