Even the KS1.3 Workbench disk contains all you need.
The shell command
will give you the amount of occupied and free space for any mounted device. Refer to http://wiki.amigaos.net/wiki/AmigaOS...Reference#INFO
I have a vague feeling that many AmigaDos commands are referenced in the original manuals so if you still have them you might want to give them at least a cursory reading. There's lot of useful stuff there.