I was also looking to do this: make the “main image” larger on the single listings page (without making all thumbnails bigger). Here is a solution. Dave R informed me that the relevant code is now in /core/templates-ui.php on line 381. The solution involves checking if the larger size exists and if we’re on the single listing page (does the image link to a larger image or to the single listing url). If either of these is false, use the thumbnail as before.
Starting from line 381, replace the next 9 lines with this:
if ( wp_get_attachment_image_src( $main_image->ID, 'medium' ) && $args['link'] == 'picture' ) {
$main_img_special_size = 'medium';
} else {
$main_img_special_size = 'wpbdp-thumb';
}
$image_img = wp_get_attachment_image( $main_image->ID,
$main_img_special_size,
false,
array(
'alt' => get_the_title( $listing_id ),
'title' => get_the_title( $listing_id ),
'class' => $image_classes
)
);
If you want to use another size besides ‘medium’, just replace that in the 2 places it appears, with your own custom image size inside the single quotes. Remember to save a copy so you can re-implement it with the next upgrade (unless Dave adds “Custom main image size for single listings” as an option).
Hope it helps someone!
T.