Based on the data type size_t why 5? If because of the currently used values I would argue that we should size the char array to for any value valid for the data type.

Because we know the value of size. I agree that it's fragile though.

I'm not aware of a macro returning the maximum length of a string representation of a type, (but if there's one I'm more than happy to use it!).
Otherwise, I can assume that for the foreseeable future we will have systems that have at most 64bits to represent size_t and use 21 for the array:

In [0]: len(str(2**64))
Out[0]: 20
# + null-terminator

Does that match your suggestion?

That sounds better to me 👍