I will be assuming that you are self-hosting the .ORG and not running a wordpress.com site.
In your theme functions (functions.php), use ctrl+f shortcut to find the following:
add_image_size( 'archive-thumbnail', 470, 140, true );
add_image_size( 'single-thumbnail', 470, 260, true );
add_image_size( 'featured-thumbnail', 750, 380, true );
add_image_size( 'slider-nav-thumbnail', 110, 70, true );
Change the figures in bold to the dimensions of the images that you wish to use in your slider. By doing this, you stop your theme automatically resizing images to the default 750x380 when selecting them as a featured image. You must re-upload new images as featured images for them to be cropped at the new dimensions.
You may then need to change the size of the slider container, but I am not 100% on how to achieve this to a specific size. Personally, I used this to extend the slider to full page width (940x397). This was achieved by using the following CSS: