This is a very healthy and simple dish that can be made with just a frying pan/skillet with a cover. It really does not matter how much bok choy you use. Measurements should NOT be carefully observed because it really depends on preference.
This dish can be served with noodles or rice, depending on what you like. If you're on a low-carb diet, this could serve 2 people. If this is going to be your main dish and you're not having any carbs, try adding some sliced mushrooms with the tofu. This will add some moisture to the pan for steaming too.
For the bok choy, I like the baby bok choy because there's no chopping necessary. Larger ones need to be chopped into one-inch pieces. This recipe works with silken soft tofu too but it is more difficult to handle. Sesame oil is optional because it's not a common thing for people to have in their kitchens.