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.