Discovered the answer myself. Pays to look deep into the reference manual
Because I have a standard monitor of 1920 x 1080, I just needed to un-tick the "Enable HIGH DPI Mode" option in Options>General. Now Studio One fills the screen maximised.
Piggybacking on your response, the screen resolution definitely needs to be correc, and often, all you need to do from there is right-click on the application's icon to select the "Maximized" option next to the "Run" text. Hope that helps!