Image size for posts - what image size to use for Sparkling theme

I’ve noticed on the demo for Sparkling you use 3 different images per post - 1920 x 600, 750 x 410, and 60 x 60. When I set my feature image to show up in the slider it only pulls from that image - how do I utilize different sizes like you have done?

For posts that are added on slider I am using images that are 1920 by 600 pixels and they are resized to 750 by 410 pixels to be displayed above posts and 60 by 60 pixels to be displayed in popular posts widget.

Images inside slider are not resized and are displayed as is. Other images are cropped/resized, so make sure to use the right size images to start with and WordPress will do the rest.