To change attributes of the popup window, look in the MAIN.CSS file for the following. Yours will not look exactly same as this.
Here is my current popup:
http://67.20.92.91/store/Bomber.html
.images-preview {
position: absolute;
top: 100px;
right: 100px;
bottom: 100px;
left: 100px;
background: #faf9db none;
border: 5px solid #291f16;
overflow: hidden;
min-width: 700px;
max-width:702px;
min-height: 400px;
max-height: 500px;
margin-left:auto;
margin-right:auto;
z-index: 4000;
}