Filtering out data on page load using angular4 + firebase?

Question

Angular

Good afternoon, the task is such that in different columns it is necessary to display different data from firebase depending on their 'size' database
structure
items
-KhDCDkRWYO7Kwgo85XI
size: "small"
text: "First text"
-KhDCG6i4eS8VrUrgu4Z
size: "medium"
text: "Second text"
html file
ul
li class="text" *ngFor="let item of items | async"
{{item.text}}
/li
/ul
and items component
: FirebaseListObservable;
item: FirebaseObjectObservable;
sizeSubject: Subject;
constructor(private http: TransferHttp, af: AngularFire) {
this.sizeSubject = new Subject();
this.items = af.database.list('/items', {
// query: {
// orderByChild: 'size',
// equalTo: this.sizeSubject
// }
});
}
filterBy(size: string) {
this.sizeSubject.next(size);
}
How can I display in ngFor all elements with 'size' = 'small' for example. The commented lines help organize the filter when clicked, but initially then it does not display anything
Thank you very much in advance