1. Go to Style.css Editor of Your Theme. ( It resides in Control Panel-> Appearance/editor/style.css Or if using Child Theme , No need to Say ?? )
Find
.featured
Change Height and width there as your needs .
2. Better open your style.css (for it , Go to your directory/wordpress/content/theme/simple-catch)in your code editor . .featured resides at 765 line. Directly Go there and change.
Your Slider size will be changed.
For Default settings of Picture , You have to go to function.php file . Search Custom featured Image and change the dimensions.
Thank You
