INT 21 - DOS 1+ - GET FILE SIZE FOR FCB
        AH = 23h
        DS:DX -> unopened FCB (see AH=0Fh), wildcards not allowed
Return: AL = status
            00h successful (matching file found)
                FCB random record field filled with size in records, rounded up
                to next full record
            FFh failed (no matching file found)
Note:   not supported by MS Windows 3.0 DOSX.EXE DOS extender