The BOL topic "Char and Varchar" says:
When n is not specified in a data definition or variable declaration statement, the default length is 1. When n is not specified with the CAST function, the default length is 30...."
In the Enterprise Manager you do specify n. 50 is a suggested length. It does not let you to select nothing for n, posts a message that n must be from 1 to 8000. Enterprise Manager is a client application which may have any restrictions specified by its programmers (Microsoft in this case) where TSQL statements and its rules are applicable to server software SQL Server.