The function takes an input string and returns the length of the string as an English word.

[CODE]:
```python
def f(x):
    num_to_word = {
        1: "one", 2: "two", 3: "three", 4: "four", 5: "five", 6: "six", 7: "seven", 8:"eight", 9: "nine",
        10: "ten", 11: "eleven", 12: "twelve", 13: "thirteen", 14: "fourteen", 15: "fifteen", 16: "sixteen",
        17: "seventeen", 18: "eighteen", 19: "nineteen", 20: "twenty"
    }
    return num_to_word[len(x)]
```

Please note that this approximation is based on the provided input-output pairs so far. If the function has more complex behavior, providing more inputs for testing would help refine the approximation.