What's the "perfect" image size?

I know the documentation suggests sizes, but when I use those sizes I get white outlines on the slider. What’s the perfect size for the slider to fill it completely? I’d like to know so that I can tailor my slider images to be that exact size.

Here are quote form documentation:

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.

To make sure that images in slider looks exactly like in theme demo you should use 1920 by 550 pixel image. These images can be narrower but it is recommended to use 1920 to make them full screen on Full HD monitors which is current standard for most monitors and TVs.

Height can be adjusted and there are no limits but I use 550px because I like how it looks but even 700px or more would work just fine.

Images in slider are not resized and are displayed “as is”.

Whenever I set my image to 1920 by 550 the slider looks amazing but the actual featured article image just shows a small portion of the image. Is there no way of having it scale down to fit the post?

You can change that by resizing featured images for posts to 750 x 215px. Current images are 740 by 410px.

This can be done via functions.php where current image size is different like this

add_image_size( 'sparkling-featured', 750, 410, true );

and you can replace it to

add_image_size( 'sparkling-featured', 750, 215, true );

Now these images will be resized in size but not cropped because it will have the same aspect ratio as slider images.

Afterwards you will have to regenerate thumbnails and you can do it by using plugin such as this.

Changes can be made via Child Theme as well but some tweaks might be required.