cant login to linux

ok ill keep it short. ive been kinda flooding the forum with questions and im sorry. but anyway this is what happened

ubuntu;s default gui or windows manager is gnome. i like kde beter so i manually installed it via the command sudo apt-get install kubuntu-desktop. ok cool restart works great. res is too high. was to begin with. 1600×1200. i want it at 1280×1024. so i do that i log off then back on and now i cant login in. it goes to a black screen for a bit then back to login. i can get into terminal session but i dont know any commands to change the res. id like to change the res back. anyone know this command. and id also like to know how to fix this problem. anyone know? much appreciated!