setupWith static method
Setup the screen with the device screen Size and the size you will use. So, if you have a design with 1280 * 720. You will pass first the device screen size then the design size.
Scale.setup(deviceScreenSize, Size(1280, 720))
Implementation
static void setupWith(Size deviceScreenSize, Size screenSize) {
_deviceScreenSize = deviceScreenSize;
size = screenSize;
}