In decreasing order of size:
Liechtenstein
San Marino
Tuvalu
Nuaru
Monaco
Vatican City
I realistically think that nobody has ever heard of Tuvalu and Nuaru.
Liechtenstein is also 'doubly landlocked'...which means that not only is it land-locked, but, it's completely surrounded by landlocked countries. There's one other country that's doubly landlocked; it's here.
