This should work:
You could use:
add_image_size( 'category-thumb', 220, 9999 ); //220 pixels wide (and unlimited height)
and then to display:
if ( has_post_thumbnail() ) { the_post_thumbnail( 'category-thumb' ); }
This will create a thumbnail size of 220 by “unlimited”… Set the featured image on the post, then use the display code in your theme to show the 220x??? scaled image.