How to make popup windows larger than image?

I have the current version of Zen Cart. I have all the product files named the same, except the LARGE images have _LRG added to file name before the extension. I can click on “enlarge photo”, but when the popup window comes up, it is about 1/4 the size of the large photo image. Can someone point me to the file where this popup window is called out? Can a person add the width and height of the image + 20 pixels so image fits inside the window space. I have searched for the file on the server and have not found one. The threads on the forum are close to this, but do not specifically address this issue.
Thanks in advance for your advice!

Have a read of the complete thread

http://www.zen-cart.com/forum/showthread.php?t=105066

Be aware that there is a known problem with FireFox and FireBug causing this problem and that FireBug is aware of the issue and will hopefully have a fix before too long ...

I have this same issue and it is only in Firefox's browser. Works great in all other browsers.

Thanks to all your input. Especially nigelt74 - thanks for the complete thread link. Found cornish input in Aug, 2008 worked great.
http://www.zen-cart.com/forum/images/smilies/clap.gif