How to change the thumbnail size to a specific category?

by

How about just use conditionals: 

if ( in_category( '9' )) {
    // different size for one category
    set_post_thumbnail_size( 100, 150, true ); 
} elseif ( in_category( array( '5', '7' ) )) {
    // different size for multiple categories
    set_post_thumbnail_size( 150, 200, true ); 
} else {
    // default size
    set_post_thumbnail_size( 328, 228, true ); 
}

Reference: