fixed image size on view large image

my image is pretty big. how could i specify a fixed size (640x480) for the pop up window when they press view large image button? which file should i edit?
thanks

I hope I understand your situation correctly.

In general, there's no need to have an image larger than the largest one you want to display. Making the visitor squeeze a large picture into a smaller one doesn't make for a better picture. In fact, it's almost always a worse looking picture and it takes a long time to download too.

That's why we do image optimization. You have a master image which is very large and detailed, and from that you create smaller images, one in each size you want to display. These smaller images look better than if you fed the visitor your master image and told them to squeeze it down.

If I haven't made it clear why you need to use image optimization, the link in my signature might help. If you'd like to know how to implement image optimization Ryk has written the definitive tutorial at:
https://www.zen-cart.com/tutorials/index.php?article=224

Or you can download the Image Handler 2 module if you're comfortable with installing modules:
http://www.zen-cart.com/index.php?main_page=product_contrib_info&cPath=40_47&products_id=117

I hope that I understood your situation correctly, and that this helps.

--Joe