Ooh ooh a question I think a noob like me can actually help with!
I just found this out right now. What you're looking to do is change the window size. To do this, first make sure your properties bar is visible. Then click anywhere you don't have any unlocked sprites/objects. The properties bar should now reflect the properties of the layout. In layout properties, it only lets you re-size the actual layout, rather than the window itself. So now you'll want to click on "View" right next to where it says "Project Properties". It now lets you change the window size!
I know this is an old thread but I hope this helps.