Here you will learn how to increase the size of a text box in the Word Processing Designer. First go to Tools > Word Processing and click on the letter you would like to edit then click the Design button.
Find the text box you would like to design and click on it. When it is selected you will see little black dots around the box like the picture below.
If you move your mouse over any of those dots you will be able to click (left mouse button) and drag the box to the size you want. Once you are happy with your changes click on the Save icon and you are all set.