var mapSizeForPrint = [
        // in pixel
        Math.round(this.pdf.width * pixelsPerMapMillimeter),
        Math.round(this.pdf.height * pixelsPerMapMillimeter)
      ];
      ...
      var map = this.openLayersMap();
      ...
      this.mapExtent = map.getView().calculateExtent(this.mapSize);
      ...
      this.rendercompleteListener = map.once("rendercomplete", event => {
       //Do printing magic
      }
      map.setSize(mapSizeForPrint);
      map.getView().fit(this.mapExtent, { size: mapSizeForPrint });