Well, whether it’s a plugin or the theme, they all use the WordPress function get_avatar()
which has a $size
parameter as its second argument. “Avatar providers” won’t necessarily serve an image exactly in the requested size, but Avatar Privacy does because it already has the infrastructure for scaling and caching images in place. Serving an image larger than requested would be wasteful of bandwidth. If you want a larger image in a particular template, make sure that the second parameter is set to an appropriate number of pixels and all will be fine.
Regarding your second question, yes, the checkbox can be disabled using this code snippet (no “anonymous” commenter will be recorded as consenting to Gravatar use though):
\add_filter( 'comment_form_fields', function( $fields ) { unset( $fields['use_gravatar'] ); return $fields; }, 20 );
As to Media Library support, I can’t promise much more than “probably in 2020” (depends on my spare time and what else is going on).