I know this is a very basic question, but I searched the web and did not find a simple answer. I'm sure it's some kind of two or three letter command, right?
du -s directory, as in "disk usage, sum"
du -sk to show the size in kilobytes (default is to show number of 512-byte blocks), or
du -sh for a more human-friendly output. For more options, type
This is the same as
-s, but replace
1 and you can get just the content in the folder:
du -h -d 0 /path/to/file
If you want the whole volume, you should use df instead of du (path is optional):
df -H /
Off-topic: One (very) spooky thing: I happend to be just a street away from Arne Stenström.