Modal Size

To define modal size. You Need to use extra class with .modal-dialog , For Example .modal-dialog-lg for large modal

.modal-dialog-sm
.modal-dialog-md
.modal-dialog-lg
Modal Position

To define modal Position. You Need to use extra class with .modal-dialog , For Example .modal-dialog-centered for centered modal

.modal-dialog-top
.modal-dialog-centered
.modal-dialog-bottom