Hah, our engineer was just trying to figure this out with support - For custom attributes, it's a million character limit. I would imagine the default fields can all be set to the same size (I know at minimum Support can change them as they just increased the Username character limit for us).
Edit: There is defaults set though - Username for example is limited to 16 characters out of box. I've never seen any docs listing the specific limits though beyond one-off help articles randomly mentioning it for a few fields, or specifically asking support.
