Header lines are limited to 998 characters (1000 - '\r\n'). Subtract 'To: ' (4 characters) and one practical maximum upper limit is 994 characters including domain.

well the minimum length is 3, and the maximum length is 254:

http://stackoverflow.com/questions/386294/what-is-the-maximu...

I don't know, but you might be able to parse the answer out of this:

http://www.faqs.org/rfcs/rfc2822.html