How to calculate size of directory on FTP?

WinSCP (free GUI on Microsoft Windows):

You can use the du command in lftp for this purpose, like this:

echo "du -hs ." | lftp example.com 2>&1

This will print the current directory's disk size incl. all subdirectories, in human-readable format (-h) and omitting output lines for subdirectories (-s). stderr output is rerouted to stdout with 2>&1 so that it is included in the output.

However, lftp is a Linux-only software, so to use it from C# under Windows you would need to install it in the integrated Windows Subsystem for Linux (WSL) or using Cygwin or MSYS2. (Thanks to the commenters for the hints!)

The lftp du command documentation is missing from its manpage, but available within the lftp shell with the help du command. For reference, I copy its output here:

lftp :~> help du
Usage: du [options] <dirs>
Summarize disk usage.
 -a, --all             write counts for all files, not just directories
     --block-size=SIZ  use SIZ-byte blocks
 -b, --bytes           print size in bytes
 -c, --total           produce a grand total
 -d, --max-depth=N     print the total for a directory (or file, with --all)
                       only if it is N or fewer levels below the command
                       line argument;  --max-depth=0 is the same as
                       --summarize
 -F, --files           print number of files instead of sizes
 -h, --human-readable  print sizes in human readable format (e.g., 1K 234M 2G)
 -H, --si              likewise, but use powers of 1000 not 1024
 -k, --kilobytes       like --block-size=1024
 -m, --megabytes       like --block-size=1048576
 -S, --separate-dirs   do not include size of subdirectories
 -s, --summarize       display only a total for each argument
     --exclude=PAT     exclude files that match PAT

If you have FileZilla, you can use this trick:

• click on the folder(s) whose size you want to calculate
• click on Add files to queue

This will scan all folders and files and add them to the queue. Then look at the queue pane and below it (on the status bar) you should see a message indicating the queue size.