IPhone : how to change UI Button width and height dynamically with respect to image in ios

[[btn_dynamic imageView] setContentMode: UIViewContentModeScaleToFill]; // btn_dynamic.contentVerticalAlignment = UIControlContentVerticalAlignmentFill; // btn_dynamic.contentHorizontalAlignment = UIControlContentHorizontalAlignmentFill; //[btn_dynamic setContentMode:UIViewContentModeScaleAspectFit];

I was tried in these area but not working