In the case of tablets, it isnt a question of large or small, but what your style of painting/drawing is. If you like to paint primarily with your wrist (ie small precise strokes) then a smaller one works much better then a larger one. In this case a large tablet will actually tire out your hand. And if OTOH you like to make large sweeping strokes with your entire forearm, then the large format ones work better. Generally for postwork or textures, where you need controlled strokes, a size like 6x9 is a nice comfortable size to work with.

Of course the best bet is to actually try out different sizes and find which size is most suitable for you.