Change the mat-autocomplete width?

In the document for MatAutocomplete there is a property for exactly what you need:

@Input() panelWidth: string | number

Specify the width of the autocomplete panel. Can be any CSS sizing value, otherwise it will match the width of its host.

By using this property you are not in the risk of meddling with anything else inherited from cdk-overlay-pane.

For further customization you might find this property useful as well:

@Input('class') classList: string

Takes classes set on the host mat-autocomplete element and applies them to the panel inside the overlay container to allow for easy styling.

mat-autocomplete styles are inherited from cdk-overlay-pane. In order to change width use below CSS.

::ng-deep .cdk-overlay-pane {
    /* Do you changes here */
      min-width:450px;
}

You can refer here for full example: https://stackblitz.com/edit/angular-vzetqy

Just add the panelWidth property to the mat-autocomplete

<mat-autocomplete [panelWidth]="300">