Sketch’s Canvas is infinite in size; you can scroll as far as you want in any direction, so you have complete freedom on how you set up your drawing area.
If you’d like to define a fixed frame inside the infinite Canvas, simply insert one or more Artboards. For example when designing mobile interfaces, many designers create an Artboard for each screen in the app and lay them out in order of appearance.
You can view the Canvas both in a resolution-independent view where you have infinite precision, or you can turn on pixel preview and you will know exactly how every pixel will look when it’s being exported to a bitmap format (such as JPG or PNG). You can learn more about these modes in Pixel Zoom.
Note: Some effects such as blur will force parts of the Canvas to be displayed in pixel preview mode, as a blur is inherently a pixel-based effect.