Angular Firebase Checkbox Using For Loop

I'm Trying to get data from firebase to a checkbox using for loop , I've tried using dropdown list box but i need them in checkbox

DROPDOWN Method

<div class="form-group">
    <label for="size">Size</label>
    <select #size="ngModel" [(ngModel)]="product['size']" name="size" id="size" class="form-control" required>
      <option value=""></option>
      <option *ngFor="let s of size$ | async" [value]="s.key"></option>
    </select>
    <div class="alert alert-danger" *ngIf="size.touched && size.invalid">
      You have to select size.
    </div>
  </div>