did you type in "fullscreen off" with the " or without the " try fullscreen off
you can try manually adjust the screen size in the options file in the electronics arts/thesims3 options if your in game at the moment then if you hit F5 while in game you be able to bring up the options menu
to do this manually close the game then go to the electronics arts/thesims 3 there is a file named options(.ini) open that search for the resolution that is in that file then find where it says resolution , resolution for example my resolution = 1280 1024 0 where it says resolution type in your screen resolution make sure to have the spaces too in between the letters then close the document it will ask you if you want to save the changes say yes (for good measure restart the computer ) launch your game and see how it is ) the refresh rate at default is fine so that would be 0
so for example if your screen resolution is 980 860 0 beside the resolution = you type in the numbers resolution = 980 860 0
if your in game you may have to ctrl+alt+del to bring up task manager to end your game before changing the file