They get between 11-15 cm long. If your animal is pet-shop bought it´s very likely it´s already adult.
The only newts that can be housed with the one you have are others of the same species, and even then, aggression may appear between individuald. Some parings work, others don´t. Mixes with other species are definitely not a good idea, at all.
A 20 gallon is very nice. It should provide enough volume to make a good home.