Hello, Karawhereibelong,
In our demo we use https://v9m6d2m2.rocketcdn.me/demos/2/artmaxy/wp-content/uploads/sites/23/2019/06/5.jpg image 1300 × 800 px. Our image does not contain text like yours, so it can be displayed properly on all devices.
If you wish to use your image with text, there are not enough settings to customize the size for different screen sizes/devices. As previously suggested, you can use the Slider Revolution plugin.
We can observe that you have already adjusted some settings for the slider on your website, which now has a full-screen size and is displaying nicely.
Kind Regards,
8theme team