What is a good size to set my images to make them Fullscreen in a GUI?

I'm working on a mod where I need an image in a GUI to fill the screen. I've tried images that are 1920x1080 but they are way to big. I've tried rescaling the images manually but that is way too time consuming and is a pain to get right. I've also tried scaling images to fit inside the GUI box inside of MCreator itself, sometimes it works, but other times the images come out being too small. So I'm wondering if there is a specific size I can set my images to to make them fit the screen or if I am doing something wrong.