How large may included parts from other people's work be in an academic paper?
You can include any material that is published as long as you appropriately cite the material (source). If you think any phrasing made by the author(s) is key, then you can do it as a quote where the text is reproduced in verbatim. If you do not use quotes you need to rephrase the text in your own words since it would otherwise be considered as a form of plagiarism. So, just be careful with referencing and quotes if necessary. It is better to have cited one time too many than the opposite.