The function takes a string input and returns the length of the input string as a spelled-out number.

[CODE]:
```python
def string_length_spelled_out(input_str):
    length = len(input_str)
    spelled_out_numbers = ["zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine", "ten", "eleven", "twelve", "thirteen", "fourteen", "fifteen", "sixteen", "seventeen", "eighteen", "nineteen", "twenty"]
    
    if length >= 0 and length <= 20:
        return spelled_out_numbers[length]
    else:
        return "Length is greater than 20."
```

Let me know if you need more tests or if you are satisfied with the results.