What do you mean by "How do I change the size of the box?"?
Do you mean you want to change the size of the family member's boxes or the family tree size?
For it, you can't change the size of family member's boxes. For the family tree, you can clearly see there's a little telescope icon in the right, there's two options that allows you to use, "-" and "+", it will let you zoom your family tree size, and the family member boxes will get zoomed up.
P.S: Hope that helps, if I mistake this question for your answer, I may try another chance to reply it. ~ Daniel
