Regarding length... I suggest you check with your supervisors. They're the ones who will be reading through it and they may have a preference about format/how long. There is defo no correct approach. So seeing what they want may help. If they have no preference you could set yourself a rough framework (e.g., 1000 words including a background, method, and references - that was the length of my proposal). The RQs may only be a few hundred words, if that.
Regarding their specificity... if your project is quantitative then I think the more specific you can get your questions the better. You might have a broad overarching aim, and then some RQs that will address that aim. The RQs might start out a bit fuzzy but then as you come to write more of the background and method, you can fill in the gaps/edit them.
As I am finding out, things like research questions often change later, as the PhD goes on... but it helpful to get something pinned down as a starting point.