Yes, this is possible- First, you'll need to click on the "Options" button on the bottom of your screen. A lot of options are going to appear, one of them being "Box width" with a slider that allows you to make the boxes wider or smaller. You can also change text size, box color and text color.
However, I believe that this is merely a cosmetic change and won't be transferred with the tree if you share it or open it from another account - everyone who opens the tree on their own device will have to adjust box size for themselves.
Hope this helps!
