- (CGSize)getImageSizeWithURL:(NSURL *)url
{
    CGImageSourceRef imageSource = CGImageSourceCreateWithURL((CFURLRef)url, NULL);
    CGFloat width = 0.0f, height = 0.0f;
    
    if (imageSource)
    {
        CFDictionaryRef imageProperties = CGImageSourceCopyPropertiesAtIndex(imageSource, 0, NULL);
        if (imageProperties != NULL)
        {
            CFNumberRef widthNum  = CFDictionaryGetValue(imageProperties, kCGImagePropertyPixelWidth);
            if (widthNum != NULL) {
                CFNumberGetValue(widthNum, kCFNumberFloatType, &width);
            }
            
            CFNumberRef heightNum = CFDictionaryGetValue(imageProperties, kCGImagePropertyPixelHeight);
            if (heightNum != NULL) {
                CFNumberGetValue(heightNum, kCFNumberFloatType, &height);
            }
            
            CFRelease(imageProperties);
        }
        CFRelease(imageSource);
        NSLog(@"Image dimensions: %.0f x %.0f px", width, height);
    }
    return CGSizeMake(width, height);
}

调用只需要

CGSize siz = [self getImageSizeWithURL:[NSURL URLWithString:@"http://h.hiphotos.baidu.com/zhidao/pic/item/6d81800a19d8bc3ed69473cb848ba61ea8d34516.jpg"]];