Maximum length of a domain name without the http://www. & .com parts

Each label may contain up to 63 characters.

"URI producers should use names that conform to the DNS syntax, even when use of DNS is not immediately apparent, and should limit these names to no more than 255 characters in length."

https://www.rfc-editor.org/rfc/rfc3986

"The DNS itself places only one restriction on the particular labels that can be used to identify resource records. That one restriction relates to the length of the label and the full name. The length of any one label is limited to between 1 and 63 octets. A full domain name is limited to 255 octets (including the separators)."

https://www.rfc-editor.org/rfc/rfc2181

The full domain name may not exceed a total length of 253 characters in its external dotted-label specification.

http://en.wikipedia.org/wiki/Domain_Name_System

If you are getting anywhere close to 253 characters, I think you should look for a shorter domain name...