I'm thinking たかだか髪 means roughly "very long hair". If so, I imagine the sentence would translate to "Very long hair grew about to that point." But, that doesn't make sense. "Hair grown like that is very long." might make more sense, but would be another pointless and obvious statement that a writer would be unlikely to write. The context, where the speaker is pondering the state of his friend's hair, would warrant something more like "His hair has grown quite long." I'm not sure, though, especially with the ending で.