Matt,
Right now we don't have the greatest way to do that from the backend component, but for now you can edit the following file to change the dimensions.
In components/com_jfbconnect/libraries/profile/facebook.php around line 214, you will see the following two lines of code where you can change the 300 to 800:
$width = $params->get('width', 300);
$height = $params->get('height', 300);
Please note, that not all Facebook profile pictures are going to be that large, so you may still get some pixelation, but this can get you closer. I will also put a task in for our next release to be able to set this easier from the backend, instead of needing to edit code.
Thanks
-Melissa