Under ‘general settings’ there is a section, top right, called ‘Blog Picture / Icon’ which mentions uploading a picture ‘to be used as your blog image across WordPress.com.’ What size should this be please, and where exactly in wordpress.com would it be used? Thanks!
That link gives you the info you are asking about and where and how to make it visible (it takes some time to show up).
As the picture in a nav bar, it will be very tiny: 16 px. Square. So as you are making the picture you want for the blavatar, test it at that very small size. Do you have a program to draw or process images so you can test how it will look?