Image Display Size (single Image When Clicked On)

I'm on a slightly older version so I'm not after an answer from Tim, but I was wondering if anyone else can assist.

When a user clicks on one of the gallery pictures a larger version of that picture is displayed - with left/right arrows on either side options above for Compare, Slideshow, Buy, Favourite etc.

I'm just trying to find where the actual display size of the image on this page can be altered.  Ideally I'd like to make it a bit larger than it currently is but I can't see where the size can be changed.

I don't believe you can change the image size after upload, but before uploading again, in admin click settings>photo settings make you adjustments and save. I believe also that your max image size must be larger than your target viewing size. IE a 2600 pixel image must be that same size or smaller, itt can not be bigger. see attachment

Thanks Vance.

I've actually already got the 'Large Max Size' width set to 1920 and the height to 1600.  This is an example of what I'm seeing on a 1920x1200 monitor (browser full-screen).  

Then that is it, thats as big as it goes.