Wordpress - Specify image dimensions

In your theme's functions.php file you can add

add_image_size( 'thumb', 600, 350, true );
add_image_size( 'thumbnil', 600, 350, true );
add_image_size( 'medium', 600, 350, true );
add_image_size( 'large', 600, 350, true );
add_image_size( 'post-thumbnail', 600, 350, true );

add_image_size( '{custom-image-type}', 600, 350, true );

For more information have a look at http://codex.wordpress.org/Function_Reference/add_image_size