You have to think about this in terms of what's transmitted on the wire, so characters count, spaces count, and so do CRLF characters. I count 291 bytes in the header below, if you include a standard blank line which seperates the headers from the content. I pasted into a text file and ran the wc command.
HTTP/1.1 200 OK
Date: Tue, 25 Oct 2011 05:21:00 GMT
Server: Apache/2.0.59 (rPath)
Last-Modified: Tue, 25 Oct 2011 05:20:45 GMT
ETag: "66ae-58c-b38e6540"
Accept-Ranges: bytes
Content-Length: 1420
Vary: Accept-Encoding
Set-Cookie: testcookie=helloworld
Content-Type: text/html; charset=UTF-8
Cirrostratus