2> /dev/null would actually work, as long as you add it as part of the find command itself, i.e., before the pipe.
For example, this will work:
find -type f -printf "%s %p\n" 2> /dev/null | sort -nr | head -n 20
With tcsh, there's no straightforward way to do the same, but man tcsh mentions a workaround:
The shell cannot presently redirect diagnostic output without also redirecting standard output, but '(command > output-file) >& error-file' is often an acceptable workaround. Either output-file or error-file may be '/dev/tty' to send output to the terminal.
For a plain find command, this gives:
(find > /dev/tty) >& /dev/null
When using pipes, in contrast to bash, you have to place
> /dev/tty after the last pipe to avoid the Ambiguous output redirect. error message.
This will work in tcsh:
(find -type f -printf "%s %p\n" | sort -nr | head -n 20 > /dev/tty) >& /dev/null