function resizeModal() {
    var viewportHeight = $(window).height();
    var modalHeight = $(".modalPopup").height();
    var maxModalHeight = viewportHeight - (.25 * viewportHeight);

    if (modalHeight < viewportHeight) {
        $(".modalPopup").css("height", "auto");
    }
    else if (modalHeight > viewportHeight) {
        $(".modalPopup").addClass("scrollableModal").css("height", maxModalHeight + "px");
    }
}

$(document).ready(function() {
    resizeModal();
});

$(window).bind('resize', function() {
    resizeModal();
});