@Component({
        ...
        animations: [
            trigger('appear', [
                transition(':enter', [
                    style({
                      //style applied at the start of animation
                    }),
                animate('300ms ease-in', style({
                    //style applied at the end of animation
                }))
                ])
            ])
        ]
    })
    class AnimComponent {

    }
]

The <div> in this template grows to 50px and then 100px and then shrinks back to 20px when you click the button.

Each state has an associated style described in the @Component metadata.

The logic for whichever state is active can be managed in the component logic. In this case, the component variable size holds the string value "small", "medium" or "large".

The <div> element respond to that value through the trigger specified in the @Component metadata: [@size]="size".

@Component({
  template: '<div [@size]="size">Some Text</div><button (click)="toggleSize()">TOGGLE</button>',
  animations: [
    trigger('size', [
      state('small', style({
        height: '20px'
      })),
      state('medium', style({
        height: '50px'
      })),
      state('large', style({
        height: '100px'
      })),
      transition('small => medium', animate('100ms')),
      transition('medium => large', animate('200ms')),
      transition('large => small', animate('300ms'))
    ])
  ]
})
export class TestComponent {

    size: string;

    constructor(){
        this.size = 'small';
    }
    toggleSize(){
        switch(this.size) {
            case 'small':
                this.size = 'medium';
                break;
            case 'medium':
                this.size = 'large';
                break;
            case 'large':
                this.size = 'small';
        }
    }
}