The size (375, 812) is a common reference size for designing responsive user interfaces, especially for mobile apps. These dimensions are based on the screen size of the iPhone X, which has a resolution of 375 points wide by 812 points tall in logical pixels. This size is often used as a standard reference for designing layouts because it represents a modern, high-resolution smartphone screen.
To make your Flutter app responsive across different screen sizes and resolutions, you can use the flutter_screenutil package. This package provides a simple and effective way to handle screen size variations by allowing you to specify design dimensions relative to a reference screen size.