To access your Slideshow Settings go to Flotheme – Sliders. The settings are global and will affect all the Slideshows, the options include autoplay, the timing of autoplay, including slideshow arrows and bullets, padding and the Slideshow image sizing and much more:

An important option is the slider height on the for the mobile devices (screen width < 641px). The mobile height should be proportional to the slideshow image height & width and the device screen width.

i.e. for iPhone 6 the screen width is 375px , thus the mobile_image_height = device_screen_width*image_height/image_width

You can use the formula above to set the correct mobile slider height depending on your image sizes.