Hey Siddharth,
Sorry for the confusion. The problem you’re facing is aspect ratio mismatch. In case you don’t know yet, it’s like placing a rectangular photo to a square frame. In order for the photo to fit, you need to cut it. With that said, it’s not possible for a Fullscreen slider to respect the aspect ratio of your image because you can never change the aspect ratio of the device’s screen.
The slider layout that you need is Auto. Since the widely used aspect ratio nowadays is 16:9, using an image with that aspect ratio like 1280x720 would appear fullscreen in most devices. For more details about Slider Layouts, please see https://www.themepunch.com/revslider-doc/slider-setup/#slidelayout
I’d also recommend that you check the tutorial about Grid Sizes in ThemePunch’s Youtube channel. See https://www.youtube.com/watch?v=OIJShHQlMx0
Thanks.