You can grow but not shrink a UFS filesystem if you manage to
increase the size of the partition it lives in, with the
/usr/lib/fs/ufs/mkfs -G -M /current/mount /dev/rdsk/cXtYdZsA newsize
Specifying the current mount point and raw device as well as the new
size in 512 byte blocks.
You can do this even when the filesystem is mounted and in use.