You can change your monitor resolution back by using the Monitors & Sound control panel.
If you have experienced other issues since installing Mac OS 8.5 please read the About Mac OS 8.5 Read Me and Installing Mac OS 8.5 Read Me that come with Mac OS 8.5.
This information is from Article 58129: "
Mac OS 8.5: About Mac 0S 8.5 Read Me"