@media (max-width: 500px) {
    .modal-dialog {
        width: 350px !important;
    }
}

@media (max-width: 400px) {
    .modal-dialog {
        width: 300px !important;
    }
    .col-xs-6 {
        width: 100% !important;
    }
}

@media (max-width: 340px) {
    .modal-dialog {
        width: 250px !important;
    }
}