How do I change the image style for avatars displayed in comments
You can modify the image style used for displaying avatars in comments by editing the "compact" display mode for user accounts.

The setting can be found at /admin/config/people/accounts/display/compact. Or by navigating the administration interface:


Admin -> Configuration
Account Settings
'Manage display' tab
'Compact' sub tab.
On the 'Picture' row, click the edit field settings button (Cog image) located on on the right.
Then select your image style.