This is not a subject matter of mirc v6.16 or any version.
If you are not connected to any server type /nick (exceeding 50 chars)
You will see your nick will be changed to a nick with the length of 50 characters. The IRCd you use has a certain limit of nick length.
For example I use DALnet:
DALnet has a support for 30 or 32 characters if I am not mistaken. Meaning using DALnet's IRCd (bahamut) you can only use nicks equal to 32 or less than 32 characters.
How ever if you have your own IRCd say client server running you can edit the ircd.conf file and change the nick length variable setting to the one which your prefer.
Every different network has a different value of nick length set this doesnt mean mirc's versions don't support those lengths.