When we talk about screens in Fluid UI we are talking about the device screen you are designing for. You can read all about choosing the right screen resolution for your app design here.
You can choose which device screen resolution you are designing for when you create a new project and by choosing project settings in the home menu.
A page in your project corresponds to a page in the app you are designing. You can change the page's height, width, orientation and set it as the starting page for when you're previewing your app prototype.
Next: Read about managing page settings in your projects.