Compatibility: 1.0+
Expects: AH    23H
DS:DX address of an opened FCB
Returns: AL    0    if file was found
0ffH if file was not in current directory
Info: Sets the randomRec field of the FCB to indicate how many records
of recSize bytes are contained in the file.

Be sure to set recSize before calling.  If you set recSize to 1,
then RandomRec will hold a 4-byte value of file size in bytes.

Notes:   You may find it easier to determine the file size by using 3dH
Open Handle followed by 42H Lseek Handle (with AL=2).

  A faster way is to use fn 4eH (find file).

  The fastest way may be to read the directory.  See INT 25H.

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DOS Fn 23H: Query File Size via FCB