The function takes a string as input and returns the length of the input string in word forms (e.g., "zero" for 0, "one" for 1, and so on).

[CODE]:

```python
def string_length_in_words(input_str):
    num_to_words = {
        0: "zero", 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"
    }
    length = len(input_str)
    return num_to_words[length]
```

Let's run the original function f(x) on a few more inputs to further validate our hypothesis.

COMMAND: PYTHON(./temp/function.py "house" "car" "train" "plane")