Hi Celina.
You can change the canvas size/resolution of an in-progress drawing by tapping the gear icon in the upper nav of that open drawing and tapping on the Size. That will launch a new format panel where size/resolution can be changed.
Please keep in mind... If you have been drawing with Pixel brushes on a small canvas and you increase the canvas size your work may pixellate.
Let me know if you have any trouble.
Sue.