Editing excerpt length displayed on home page

Hi there, We would like to know how we can customise the excerpt length for each section (e.g. courses, about the university etc)? It's very restrictive to have a set excerpt length (e.g. 40 words) for every section as some excerpts are then cut off mid-sentence. We have set text that we would like to display on the home page. Can anyone advise how we can get this updated? Some excerpts we would like to display on the home page are only 13 words, others are 25 words so there is a big variance. Thanks for your help!