command-line find – What Are the File Size Options for ‘find . -size’ Command?

find(1):

-newerXY reference
          Compares the timestamp of the current file with reference.   The
          reference  argument  is  normally the name of a file (and one of
          its timestamps is used for the comparison) but it may also be  a
          string  describing  an  absolute time.  X and Y are placeholders
          for other letters, and these letters select which time belonging
          to how reference is used for the comparison.

          a   The access time of the file reference
          B   The birth time of the file reference
          c   The inode status change time of reference
          m   The modification time of the file reference
          t   reference is interpreted directly as a time

and show no contents when cat-ing the file.

No output of cat for a file does not imply that the file is empty as the following experiment shows:

$ truncate -s 1M foo    
$ ll foo
-rw-r----- 1 user users 1048576 Nov 15 19:28 foo
$ file foo
foo: data
$ cat foo
$ 

cat does output 1MiB worth of NUL characters, but those characters happen to be invisible in a terminal.

Your question about empty files is somewhat ambiguous. Does foo above qualify? If "empty" means length zero, then find could aid you:

find dir -type f -empty

lists all zero length files under dir. If your find doesn't support -empty, you can use -size 0 instead.

In shell scripts you may use the -s file expression which is true if file exists and is non-empty. In order to check whether a file is empty (given that it exists) use something like

if [ \! -s file ] ; then ... ; fi

Alternatively, you may use stat(1). Here with the GNU variant:

$ stat --format=%s foo
1048576

which you may use further in comparisons.