I've been there, done that... glad to know I'm not the only one that has set their screen res to something they can't display, and not known how to fix it, either.
So you are at a console? Login as root by doing:
It will prompt you for your root password.
Password: <type password> <hit enter> (The line will not move when you are typing. It took me ages to figure out where to type the password, due to that... LOL)
Now you should be root.
Make sure you are at the root of the filesytem (/), then
cd etc <enter>
cd X11 <enter>
vim XF86Config-4 <enter>(if vim doesn't work, try vi)
Use the arrow keys to get around in the file.
Find the section somewhat like this:
Virtual 1024 768
Anyway, something like that... might be different for you.
Place the resolution you would like to use first in the list, in all the sections where you find the wrong res..
When you are done, hit escape and then type :wq to write to the file and then quit.
Reboot. Everything should work.
You probably knew how to do most of that, sorry if it sounded a bit too easy.