You can adjust the width of your overall site container inside Appearance > Customize > Layout > Container.
You can use the slider to adjust the width in pixels, or directly input your number into the field.
You can set it to a percentage using CSS, however I typically recommend choosing a really high pixel value like 2000 if you’re looking for a 100% width website. Since GeneratePress is responsive, it will be 100% wide on most screens at that width.