To expand on Ryand's answer, if you're using Bootstrap.ui, this on your modal-instance will do the trick:
modalInstance.rendered.then(function (result) {
$('.modal .modal-body').css('overflow-y', 'auto');
$('.modal .modal-body').css('max-height', $(window).height() * 0.7);
$('.modal .modal-body').css('height', $(window).height() * 0.7);
});