if I am not wrong, you can't grow / and /var file system
If it is really the true that you can't grow directly using growfs,
1. break the mirror,
2. bring down to single user mode
3. create newfs on /var on the mirrored disk with the size u want
4. ufsdump over
5. boot from 2nd mirror
6. attach back
re-reading the growfs man page with a fresh mind I discovered that while growfs cannot be used on root, the other restriction that tripped me up here was that accounting was running on this server and it was writing to the /var filesystem. I stopped process accounting and extended accounting and repeated the growfs and it worked! restarted both accounting processes and all is well with the server.
Thanks to both folks who responded to my post. your thoughts were the jog my brain needed!