Click the Text Box icon on the Home Ribbon and then draw your text box on your slide.

Click on one of its sizing handles and drag until the object is the desired size.

Click inside the text box and press the arrow keys until the object is the desired size.

Click the text box, click the Resize icon on the Drawing Ribbon and then drag until the object is the desired size.

Click on the border of the text box and drag until the object is the desired size.