With a standard Xara slideshow, you can proportionally adjust the height and width of the slider, but it should not extend beyond the page width. You should not be applying ‘Stretch-Full-Width to any Xara slideshow either. You can of course increase the page width to (say) 1440px or similar, but I suspect what you are looking achieve is a slider that extends to the full browser width, regardless of the page size.
There are examples of how to code this functionality here on TG and one has the option to adopt third party sliders, such as WOW. You also need to consider what happens in a mobile variant design.
Personally, I would keep it simple, adopt a desktop width of 1440px and have your slider width 1440px to match. It then works perfectly with Export > Scale-to-fit-Width
Here are some example sliders that can be extended to a page width :
https://initiostar.co.uk/demo/zNivo/
Here is a WOW slider (slightly modified code) :
https://initiostar.co.uk/demo/wow-animation/
And a Xara Full-Browser-Width Slider :
https://initiostar.co.uk/demo/Pan/index.htm - this is tad tricky but works up to 4K screens. The transition is a Cube Rotate > Vertical.
Others here on TG might have other suggestions too.
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