if you don’t mind applying this change to all large alerts then target this bit of css
@media (min-width: 992px) {
.modal-lg {
width: 900px;
}
}
adjust the width as you need - above is the default that anvil uses for large alerts.
something like
@media (min-width: 768px) {
.modal-lg {
width: 90vw;
}
}
edit - forgot the braces