Custom marker image on swift (MapKit)

I am using a custom marker on MapKit

How do I change the width and height of the image on a custom marker?

my code:

func mapView(mapView: MKMapView!, viewForAnnotation annotation: MKAnnotation!) -> MKAnnotationView! {

    if (annotation is MKUserLocation) {
        return nil
    }

    let reuseId = "test"

    var anView = mapView.dequeueReusableAnnotationViewWithIdentifier(reuseId)
    if anView == nil {

        anView = MKAnnotationView(annotation: annotation, reuseIdentifier: reuseId)
        anView.image = UIImage(named:"x2.png")
        anView.canShowCallout = true
    }
    else {
        anView.annotation = annotation
    }
    return anView
}