I would say a good size tank would be about 75 gallons, so---this is a rough conversion-- about 170-200 liters. It is good to have some algal growth established in your tank for him to graze on, a lot of difficulty that people have with keeping tangs is that they get sick because they aren't getting enough to eat---so this leads to head erosion and so forth.
Starfish are typically easy. They eat stuff like clams and snails that they can wretch open. All you really have to do if feed them these every now and then, and keep a lid on your tank.