Your template has a width of 80% for modals for the following element "body.modal-open .modal". I can't see which file since you have JCH Optimize turned on but that should get you closer. If you want to change the width and don't want to affect your entire website styling, you can target our module more specifically with something like the following (you'll also have to update the left style, etc to adjust for the new width)
body.modal-open .modal.sourcecoast {
width: 50%;
}
Modal pop up is too wide
