If you put a pacific blue tang in a 30 gallon tank will it still grow to full size?

Yes, and almost any fish will not "stay small" in a small tank. Blue tangs need a very large space, as in many feet, to move in order to live for many years. It is generally recommended that a blue tang be placed in no smaller than a 75 gallon tank. Also, no tang can be kept in 30 gallons. Some alternatives to the open water nature of a tang would be a small group of blue reef Chromis (Chromis cyaneus).