255/55 is stock aspect ratio.

255 is width. 55 is sidewall. its best to stay relatively narrow for off roading in snow and lose road.

you can go wider, but its not needed. you can go taller but you need to start thinking suspension lift. if you go with stock sizing you can have a more aggressive tire without having to modify suspension.

personally i am going 2" on a lift with a taller tire. but from teh sounds of what you are looking to do the Nitto's will do what you need.