last -w should do the trick for you. I don't have any long usernames to test this with but the man page suggests it should work.
You can also use
last -a to list hostnames as the last field and not truncate them or
last -i to display them as IP addresses instead of hostnames.
These are local users to your system so you could grep them out of your
/etc/passwd file if the above options don't work. You should only run into problems doing that if you have two users that share the same first 8 characters.
Failing all of that, you could try
strings /var/log/wtmp. This data will be somewhat more difficult to interpret but it should contain the full usernames and hostnames.