The most common is the traditional RWT, but travel size (traditional deck on the right for reference):
And then a lot of oracle decks have their own mini variation these days (sizes vary based on the deck).
The WU travel set has a mini guidebook as well. Great for beginners but you could save even more space just carrying the cards.
If you really want to push it, there are also some mini Tarot decks, but you start to even get into keyring size at that point. You need good eyesight to work with a deck like that!