This is a commonly recurring problem for people, and has just happened to me ._.

I'm stuck in 800x600 fullscreen, can't find the options to get out of it because they're hidden, and can't find any config files to change the resolution that way.

What do I do?

you may want to try these:
(1) Change your desktop resolution to 1024x768 or a 4:3 compliant resolution
(2) Start LastChaos and Login. At the Character select screen click the Options button.
(3) Uncheck "Fullscreen" and click Ok, then Shutdown LastChaos
(4) Return to your native (normal) desktop resolution and then Restart LastChaos
(5) From the Otions Menu on the Character Select Screen, choose a 16:9 widescreen compliant resolution such as 1280x720, etc.

that was taken from the below link:
http://www.aeriagames.com/forums/en/viewtopic.php?t=1170810


i just thought that might work or check the above link to check for more.

Thanks for the reply.

I still could not see any of the options screens, as I had no characters and was starting fresh.

I had to reinstall to fix the problem.

good luck reinstalling!