Hi there,
This is due to your WordPress theme / skin using another copy of prettyPhoto javascript that we also use for the popups. The one you have with your skin overrides the copy we include at the following location which is also customized to make 360 product viewer re-sizing work in the popups.
/new-site/wp-content/plugins/webrotate-360-product-viewer/prettyphoto/js/jquery.prettyPhoto.js.
If you are familiar with your WP skin implementation, you can just remove prettyPhoto script from your skin js scripts and use the one above instead (it's a pretty recent version of prettyPhoto). Please note that prettyPhoto's CSS scripts can be left as-is as they do not interfere.
The other quick fix is to open this file below and change line 22 and 23 to your fixed 360 image dimensions when loading inside the popup.
/wp-content/plugins/webrotate-360-product-viewer/viewloader.html
.viewerbox
{
margin: 0;
padding: 0;
width: 300px;
height: 300px;
}
I hope this helps.