- (void)viewDidLoad {    
    [self.webView.scrollView addObserver:self forKeyPath:@"contentSize" options:NSKeyValueObservingOptionNew context:nil];
}

- (void)dealloc {
    [self.webView.scrollView removeObserver:self forKeyPath:@"contentSize" context:nil];
}

- (void)observeValueForKeyPath:(NSString *)keyPath
                      ofObject:(id)object
                        change:(NSDictionary *)change
                       context:(void *)context
{
    if (object == self.webView.scrollView && [keyPath isEqual:@"contentSize"]) {
        // we are here because the contentSize of the WebView's scrollview changed.
        
        UIScrollView *scrollView = self.webView.scrollView;
        NSLog(@"New contentSize: %f x %f", scrollView.contentSize.width, scrollView.contentSize.height);
    }
}

2017/7/14 更新:
碰到KVO无限触发可以用js方法获取高度

- (void)webView:(WKWebView *)webView didFinishNavigation:(null_unspecified WKNavigation *)navigation {
    [webView evaluateJavaScript:@"Math.max(document.body.scrollHeight, document.body.offsetHeight, document.documentElement.clientHeight, document.documentElement.scrollHeight, document.documentElement.offsetHeight)"
              completionHandler:^(id _Nullable result, NSError * _Nullable error) {
                  if (!error) {
                      NSNumber *height = result;
                      // do with the height

                  }
              }];
}