I know window size shouldn't be changed for anonymity reasons, but what if the page has already loaded?
Does it still affect anonymity?
edited due to @DJCrashDummies comment
http://ip-check.info/ has proven this to be insecure. That is, window size change can be determined even after the page has loaded completely. As a proof of concept, try the code from https://stackoverflow.com/a/26879317/1587329, and resize your browser width after the page has loaded (with JavaScript disabled). You will see that it is dynamically adjusted.
This could unfortunately yield the browser width (and height) for an enterprising attacker.