I put the subject (as in chemistry/biology/English whatever), then a shortened version of the title of my project.
My project is related to geology, and its title is 'What would be the impacts of an eruption of the Yellowstone supervolcano on the USA?', so on the UCAS form in the EPQ subject field I just put 'Geology: Impacts of Yellowstone eruption'. It has to be fairly short or it won't fit in the box...