I went to try out the game but then my screen turned small when I opened it, I went on the game options to try to re-size it but all it did was pass my screen. I could not click on the options anymore because it was off screen so instead I exited the game. When I tried to open the game again it says "Full Screen Error", is there a way to fix this or do i have to re-install the game?

Definitely do not need to reinstall.

In the Shaiya directory there is a config.exe file. Run that and change the screen resolution.

If you have file extensions turned off, there will be 2 files called config, one of them will have the Shaiya icon - that's the one you need to run.