WordPress Fancybox Resize Large Image

What you will need to do is edit your theme’s functions.php file to add a new image size. Scroll to the bottom and inside the php tags enter:

add_image_size( 'fancybox-full', 800, 9999 );

This will create an uncropped image 800px wide, whenever an image is uploaded.

You will then need to edit the anchor for your images to link to this new image size. Try using wp_get_attachment_image_src() – use fancybox-full as the image size argument, not sure how yours currently gets the attachment ID, so if you need further assitance, let me know.

You will find this doesn’t currently work with the images you have already uploaded. Try using the Regenerate Thumbnails plugin.

Hope this helps.