Hi Sean,
Please try adding the code that @Rad suggested in X > Theme Options > JS.

In case you are still getting the smaller images, please add this to the functions.php file of the child theme:
if ( ! function_exists( 'x_featured_image' ) ) :
function x_featured_image( $cropped = '' ) {
$stack = x_get_stack();
$fullwidth = ( in_array( 'x-full-width-active', get_body_class() ) ) ? true : false;
if ( has_post_thumbnail() ) {
if ( $cropped == 'cropped' ) {
if ( $fullwidth ) {
$thumb = get_the_post_thumbnail( NULL, 'entry-cropped-fullwidth', NULL );
} else {
$thumb = get_the_post_thumbnail( NULL, 'entry-cropped', NULL );
}
} else {
if ( $fullwidth ) {
$thumb = get_the_post_thumbnail( NULL, 'full', NULL );
} else {
$thumb = get_the_post_thumbnail( NULL, 'full', NULL );
}
}
switch ( is_singular() ) {
case true:
printf( '<div class="entry-thumb">%s</div>', $thumb );
break;
case false:
printf( '<a href="%1$s" class="entry-thumb" title="%2$s">%3$s</a>',
esc_url( get_permalink() ),
esc_attr( sprintf( __( 'Permalink to: "%s"', '__x__' ), the_title_attribute( 'echo=0' ) ) ),
$thumb
);
break;
}
}
}
endif;
Here are some references for the child theme:
https://theme.co/apex/child-themes
https://theme.co/apex/forum/t/setup-how-to-setup-child-themes/57
Hope this helps.