Cant change the size of my screen

Hello, im having some problems with my game. as soon as i opened it today it just kept lagging and the game eventually started closing because it said i apparently dont have any memory on my computer? i uninstalled the game and re-downloaded it. I can get in the game but now i cant change the size of the screen and its really annoying. I apply the new size i want but it keeps turning back to the old one. It worked perfectly fine until today, so i dont think anythings wrong with my computer. please help me fix this asap ): </3 <3

See if manually changing the resolution via WoW/WTF/Config would help.

SET gxResolution "1680x1050"

Change numbers to fit yours.

While at it, check your WoW folder's properties, uncheck read-only if it's checked.
For general stability, I recommend using Large Address Aware on your WoW.exe.