I have had a look in the file browser app and the External Drives app, long-pressing various items in both apps, and looking for any "menu" items in both of these apps, but I haven't found how to see the size of the card.
In Ubuntu 16.04 (on my desktop computer), for example, I can start the Disks utility (actually an application called
gnome-disks, and not
gnome-disk-utility as shown Disks > Menu > About) and see total size and % used for each storage unit (SSD, HDD, and external drives).
I can also see a drive's total space and free space from within Files (actually
How do I do it on this tablet?