ls -l, which only displays the size of the individual files in a directory, nor
df -h, which only displays the free and used space on my disks.
The command du “summarizes disk usage of each FILE, recursively for directories,” e.g.,
du -hs /path/to/directory
-h is to get the numbers “human readable”, e.g. get 140M instead of 143260 (size in KBytes)
-s is for summary (otherwise you’ll get not only the size of the folder but also for everything in the folder separately)