Restrict the lightbox image size to 1200

please visit this link : http://ift.tt/1B0R4N7

click on "product image", you can see image ,image will expand with original size [lightbox image]

I want to restrict that lightbox image size to 1200 in product view page in magento.

please see below media.phtml file

media.phtml file = http://ift.tt/1AOmrvi

help me to restrict the lightbox image size for 1200.

let me know if you need any clarifications,

thanks in advance,....