Alert width in page.scss


I am applying css to an alert using:

let alert = this.alertCtrl.create({
      cssClass: 'custom-alert',

How do you change the size of the box the alert is in - not the semi-transparent background.
The following changes the entire alert with the trans BG:

.custom-alert {
  width: 500px;

I realize I could use $alert-min-width: 60%; in the variables.scss but i am trying to do this in the page.scss.