after setting featured image. parameter size accepts thumbnail, medium, large or custom eg. array(230,234)
<?php $theattr = array( 'src' => $src, 'class' => "attachment-$size", 'alt' => trim(strip_tags( $attachment->post_excerpt )), 'title' => trim(strip_tags( $attachment->post_title )), ); the_post_thumbnail( 'large', $theattr ); ?>