The styles for the modal sizing are coming from your template.
For instance, "body.modal-open .modal" has a width of 80% of the screen in
mxgroup-new.under-development.info/templ...ank/css/template.css around line 13907.
You'll need to adjust the width there and the margin-left, so that it aligns in the middle. Here's a suggestion of some CSS to start with, but you may need to tweak it
body.modal-open .modal {
width: 40% !important;
margin-left: -20%;
}
-Melissa
Modal popup window size
