1. The size of tofu in a pack is about 11cm x 8cm x 4cm (4⅜" x 3⅛" x 1½"). I like the texture of silken tofu as it is more delicate but you can use firm tofu (momen-dofu) if you prefer. It is really your choice but do not use extra firm tofu as you will not enjoy the texture of the tofu. I have talked more about different tofu in the post Tofu With Vegetable Sauce if you are interested.
2. I had these three kinds of garnish on hand so I listed them here. But other garnish you could use are grated ginger, shichi-mi tougarashi (七味唐辛子, seven-flavour chilli peppers), momiji oroshi (紅葉おろし) which is grated daikon with dried chillies (make few holes in daikon, put chilli into each hole, then grate daikon).
4. If you have a portable cooktop, it would be ideal to cook the tofu at the table, especially if you are serving a few people. In that case, leave the tofu in a bowl full of water and when the water in the pot gets heated take as many tofu pieces as you can eat at one time and cook them in the pot. This way, you can eat perfectly cooked tofu every time.