Most of the time when I share a folder from Ubuntu, the name field is made yellow and it says "Share name too long". But even if I keep this name, it works fine. So what is it too long for? What is the limit?
There is a bug in the dialogue. Add another character and you'll notice the warning disappears!
The warning is there because some systems that can access Samba filesystems also have fairly extreme filename-length limits. For example you can connect to a SMB server with Windows 3.11 which has the same file restrictions as DOS.
Samba will still host the share but other people might not be able to see it. Similar rules apply if you stick restricted character (eg: a question mark) in the share name. Other POSIX systems will cope. Windows will fall over.