I'm trying to change the size of the actor only once because it keeps getting smaller and I don't know what The constructor is
There are many ways to limit it to a one-time action. Obviously, you have found that coding the action in the constructor is one way -- and in this case, probably the best way. The constructor is a block of code that looks like a method, but (1) does not have a return type (in most cases, it would be the 'void' in method declaration lines); and (2) the name of the block has the same name as the class. So, a Player class would have a constructor looking like:
public Player()
// or
public Player(Healthbar bar)
The parameter listing is irrelevant as long as the name is the name of the class and there is no return type given.