1440x900 is a 16:10 resolution and CK is rendered out in 16:9, which would account for the black bars (though not sure why the main menu is rendering fine).
Unfortunately, Ren'py isn't a 3D game engine so adjustable resolutions aren't really possible. Best I can advise is to use windowed mode and then just manually expand the screen size by clicking and dragging on the corners or the window.