Featured Image (Blog and Single Post) is set to 750 by 410 pixels.
Featured Slider Images. Personally I use 1920 by 550 pixel images for Sparkling Demo but you can feel free to use any other image size. Images won’t be scaled and you will get what you upload.
Please note! Images for slider is not resized and are displayed the same size you uploaded them. Other images such as featured images for posts are resized automatically as you upload them.