Select the number and choose a font size large enough to fill the page.
Although the font sizes listed on the dropdown are limited, you can type in
any number between 1 and 1638 (in half-point increments) and press Enter.
For reference, there are 72 points to the inch.
Because most fonts are designed with some "leading" (extra line spacing),
you'll probably need to reduce the line spacing of the paragraph to the
nominal point size of the font (or sometimes even less) in order to fill the
page.