From Slider settings you can set some global options related to the main Slideshow and to the Sliders used on the single gallery posts. The image below shows all available options:

To allow your images to be retina ready, you’ll want to make sure your theme is updated to the most recent version.

Once thats done, go to Flothemes > Image Sizing, and then scroll down to the mobile slideshow slides width option, and set a size here of around 700px:

With this set, your images should be ready for both retina and non-retina mobile devices.