In the middle of the Editor interface, you see your canvas, your interface design area. The canvas size is set based on the device you’re designing for. You choose a device when creating your project. Your canvas is where you will be designing your prototype’s screens.
If you want to change your screen canvas size, you will have to go to the Dashboard-> Project Settings and change the ‘Device’ to a new one, or create a custom device (see Creating a custom device).
If you have designed your prototype for a certain device and then changed it, you will see the following dialog next time you edit your project. Proto.io can automatically adjust UI items to the new device settings.
When designing Containers (see Containers), then the canvas you see is the container canvas and dependent on the container’s size that you choose. This might be different to your screen size. You can modify the container canvas size at any time through ‘Edit container properties.’