如何求出imgae在屏幕上的大小

- (CGSize)getScaleImageSize {

    float heightScale = self.mainImage.frame.size.height/_selectedImage.size.height/1.0;

    float widthScale = self.mainImage.frame.size.width/_selectedImage.size.width/1.0;

    float scale = MIN(heightScale, widthScale);

    float h = _selectedImage.size.height*scale;

    float w = _selectedImage.size.width*scale;

    return CGSizeMake(w, h);

}