Seems as if my choice of how I set the repeater up was causing the issue. I had success with the below code:
if( have_rows('grid') ):
while( have_rows('grid') ): the_row();
$image = get_sub_field('image');
$value = get_sub_field('image_size');
echo '<div>';
echo wp_get_attachment_image( $image, $value );
echo '</div>';
endwhile;
endif;
Thanks for your reply. I gave your suggestion a try and no luck. I also tested out whether defining directly worked and it does (I dropped ‘prev-landscape in and the images rendered at the correct size).