The default avatar is stored in the following location.

/components/com_comprofiler/plugin/templates/default/images/avatar

You can resize it as needed, save a copy with a new name, then select it as your default within the avatar fields parameters in CB > Field Management.

With that said I do not recommend doing this. In fact I recommend editing your avatar field setting the following new default values.

Maximum Width: 160
Maximum Height: 160
Maximum Thumbnail Width: 80
Maximum Thumbnail Height: 80
Main Aspect Ratio: Yes, with User Cropping

Our canvas layout and throughout CB is very well designed for those sizes. The above will give perfectly squared 160x160 full size and 80x80 thumbnail. In a future CB release (likely not before CB 3.x) we will be forcing specific sizes on avatar and canvas fields (they will not be changeable) so we can have a more strict design.

It shouldn't have been too big. Where was the sizing issue? We still have a few plugins that need their UX improvements and we're working through them.

The login module should be outputting thumbnail so it'd be 90x90 if using a bordered image style otherwise 80x80. I'm not seeing 180x180 in my tests, which certainly would be too big for the login module.

How are you outputting avatar in your login module? It should be output by just toggling the respective parameter on in your login module. If you're outputting it with a substitution you maybe outputting the full size avatar and is why it's so large.