Using the default settings of Facebook OAuth2 in Moodle, during login, the user's photo is downloaded in a very small resolution (50x50 px). Good picture to use in Moodle must have at least 100x100 px. There are several ways to download larger profile picture from Facebook. We can set the preferred dimensions manually or use predefined ones. When we set the photo type to large, the dimensions of the photo are 200 x 200 px. We can even set for example height to 10 000 px. Then we will definitely get the largest possible picture. It seems to me that the standard, predefined setting (large) will be sufficient for us.

I changed profile picture size to large in the Moodle default Facebook service OAuth2 configuration.

More about it on Facebook Graph API docs