Featured Image Help!

Hey fellow members, I have a question I want to ask because im having a little trouble.

Now my featured image is working but the theme I am using shows a small thumbnail on the post, and I want to change it to show a larger thumbnail.

Now here is how my functions.php is set up:

// Thumbnail sizes
		add_image_size( 'large', $content_width, 9999, false ); 	// 940 => Full width thumb for 1-col page
		add_image_size( 'medium', 620, 9999, false ); 				// 620 => Full width thumb for 2-col/3-col page
		add_image_size( 'small', 195, 195, false ); 				// Square'ish thumb floated left
		/* Slider - Heights equal at 350 */
		add_image_size( 'slider-large', 940, 350, true );			// Slider full-size image
		add_image_size( 'slider-staged', 564, 350, true );			// Slider staged image
		/* Grid - Ratio: 200:125 */
		add_image_size( 'grid_fifth_1', 200, 125, true );			// 1/5 Column
		add_image_size( 'grid_3', 240, 150, true );					// 1/4 Column
		add_image_size( 'grid_4', 320, 200, true );					// 1/3 Column
		add_image_size( 'grid_6', 472, 295, true );					// 1/2 Column

So say I want to use the thumbnail called “slider-large” 940X350 because I like that its long and thinner than the other croped thumbnails.

So I would execute that thumbnail on the post with this code: the_post_thumbnail( 'slider-large' ) but the problem is that Its going to be too wide for my post space.

Is there any way I can resize that certain slider-large thumbnail something like: the_post_thumbnail( 'slider-large' 540,350 ) or something like that is there a proper code to do that or not?

Please any help would be appreciated.