Say that the length of your dissertation is 5,000 words.
Before you even start writing you should plan to set aside a certain number of words for your introduction and your conclusion.
You should allow yourself a minimum of 600 words for an introduction but it could be as long as 1000-1200 words depending on your topic and approach to it.
You should allow yourself a minimum of 300 words for your conclusion i.e. approx. one A4 page double-spaced but, like your introduction, your conclusion could be up to twice that length.
So if you decide on the suggested minimums of 600 and 300 that is a total of 900 words which leaves you 4,100 for the main body.