Valid question. I would assume not more than 15' but no answer seems to exist in the text as I see it written on the SRD. May even be as little as 5' (i.e. the square behind you) but it seems improbable that it would be described as a 'cone' at that range rather than designating the square exactly.
Could be a 10' or 15' cone but RAW the answer seems to be missing.
Yep, no RAW answer here and since we don't even have an FAQ system anymore, it's unlikely we ever will. I'd say 10' cone is a safe bet [it's the smallest cone you can have and it's only a 1st level feat]: for anything more, you'd have to ask your DM