这方法挺实用的，直接调用就可以得到想要的size。

//将一个UIImage缩放变换到指定Size

-(UIImage*) OriginImage:(UIImage *)image scaleToSize:(CGSize)size

{

    UIGraphicsBeginImageContext(size);

    [image drawInRect:CGRectMake(0, 0, size.width, size.height)];

    UIImage* scaledImage = UIGraphicsGetImageFromCurrentImageContext();

    UIGraphicsEndImageContext();

    return scaledImage;

}