Hi Slabadie,
Each theme comes with two slider widths. If you choose to display a full width slider then the image used will cover the full screen width, provided it is atleast as wide as the screen being used. So there is no fixed width that’s recommended for the full-width slider. However, we do recommend to use a large image, atleast 1600px wide.
For the boxed slider, the slider will be restricted to a maximum width of 1140px (or 960px depending the theme width). So in this case, we recommend using an image which is also 1140px wide (or 960px).
The height of the image is less important as you can adjust the slider height directly from the theme options panel. We advise that the slider height be no greater than the image height.
Here’s a screenshot which shows where the slider height setting can be found:
https://prntscr.com/iac966
Thanks!