/nick testing_this_very_long_and_stupid_nick
* Your nick is now testing_this_very_long_and_stu

Yep, 30 characters
Im running mirc 6.16 on an Ureal ircd 3.2.1 network (mine)
I did compile it for 32 chars

mirc 6.12:
* Your nick is now testing_this_very_long_and_stu

Same problem!

I just tried X-chat from Linux Redhat Fedora, and still 30 chars long.
So if youre running unreal ircd like me, then that may be the cause.