When you’ve updated discourse to include this commit (on latest as of August 12)
https://meta.discourse.org/t/how-to-change-topic-avatar-size/20689/23?u=yuun
You should use this script to change the avatar size up in the header:
<script type="text/discourse-plugin" version="0.4">
api.changeWidgetSetting('header-notifications', 'avatarSize', '70');
</script>
Instead of the big ole mess up above.