<mat-sidenav #sidenav  mode="side" class="primary-color main-sidenav nav-
 shadow" opened="true" [@sidenavState]="sidenavState"
 (@sidenavState.done)="endAnimation()" *ngIf="layout === 'modern' ||
 forceModern">
 ....
 </mat-sidenav>

.mat-side-nav{
   width: auto !important
 }