Talk to both staff, family, friends, and, if you can, current PhD students. Ask them about their experience of planning a research project. Talk to your Masters dissertation supervisor about your thoughts and they can let you know if your plan seems too narrow or too broad. They will probably also offer some recommended reading.
Friends and family, though probably not specialists, or necessarily academic minded people, can often help you get your mind out of the details and into the broader picture.
My parents have not studied History since O-Levels were a thing, so I thought they probably wouldn't be very useful. But the thing is, they know me better than anyone.
“You should do the East India Company”, my dad said. As a kid I loved films like Pirates of the Caribbean (lets not talk about the new ones), and Indiana Jones. He knew I had a love for adventure. Historical adventure.
Having previously specialised in early colonial America, my lack of language skills and the limited number of specialists in the UK meant that it would probably be a difficult topic to pursue. . . not to mention it is also a very crowded market.
After discussions with my Masters supervisor about the kinds of sources I like to use and the themes I enjoyed researching, he ended up encouraging me to look at sea travel further afield, something my dad had been talking about for years. So I did. While I now situate my studies in the Atlantic, not really looking at the East India Company, my dad wasn’t far off. . .