The standard doesn’t define a maximum length, leaving implementation up to browsers and mail clients (See IETF RFC 2368).
Microsoft products do have set limits:
- IE GET limit is 2,083 http://support.microsoft.com/kb/208427
- Outlook express: 456 characters http://support.microsoft.com/kb/q182985/
Other browsers are likely to work up to lengths beyond that of a reasonable email body. The iPhone doesn’t have a documented limit, but works with up to 1MB of text.
Modern browsers that support data urls (everything except IE<9) should be fine.