TextArea textArea = new TextArea(); //making a TexrArea object
double height = 400; //making a variable called height with a value 400
double width = 300;  //making a variable called height with a value 300

//You can use these methods
textArea.setPrefHeight(height);  //sets height of the TextArea to 400 pixels 
textArea.setPrefWidth(width);    //sets width of the TextArea to 300 pixels