slider and functions.php

Size of the slider means? Is it the height of the slider? For more info on carousel (slider), refer carousel part.

Functions.php is empty because you’ve installed the child theme, and in child theme, functions.php and style.css comes empty, so that you can override the parent theme or you can make changes on top of parent theme.

If you want to edit it, go to appearance->editor. If you’ve activated your child theme, then you would be able to see style.css page opened by default. If you need to edit functions.php, then on the right side click on functions.php and edit it.