If you mean print to a printer, then after you click "Print tree", look in the print dialog box on the left. There are two settings you can change to "spread out" your tree - Width and Height.
Increase the Width number to spread your tree across multiple pages (tape them together after printing). If the boxes look squished even when you've increased the Width, then increase the Height to 2 (or 3).
Use the Print Preview button to see how the pages will look before actually printing them.
If you mean on the screen, just click the + or - magnifying glass icons at the bottom of the screen to make the font size larger or smaller, but you'll likely not be able to get the entire tree on the screen at the same time.