How do I get the size of a directory on the command line?

du -sh file_path

Explanation

du command estimates file_path space usage
The options -sh are (from man du):

-s, –summarize
display only a total for each argument

-h, –human-readable
print sizes in human readable format (e.g., 1K 234M 2G)
To check more than one directory and see the total, use du -sch:
-c, –total
produce a grand total