Why does the image generated by StyleKit have a constant size (image size equal to canvas size)? How can I change this?

Use the Frame tool to define a rectangle that defines position and size of your drawings.

The frame will become a parameter of imageOf... method in n your StyleKit.

Please, check out the "Example.pcvd" document, particularly the "BlueRectFramed" canvas in the "Basic" tab of the example project. The EXViewsLayersViewController subclass has a viewWillAppear: method that sets self.imageView.image:

// Swift
self.imageView.image = StyleKit.imageOfBlueRectFramed(frame: self.imageView.bounds)

// Objective-C
self.imageView.image = [StyleKit imageOfBlueRectFramedWithFrame: self.imageView.bounds];