Good question. Because who doesn’t want to put vision tests in their emails?

TO
CHANGE
FONT
SIZE,

make sure you're composing in Rich Text. Do you see a toolbar above the Compose window, with buttons like B for Bold and a blue "T" for text color? If not, just above the upper-right corner of the Compose window, click Rich Text. Now you should see the toolbar, which includes a button (a big A and a little A) for choosing size.

Either click the button, choose a size and start typing, or select a block of text you already typed (or just one word), and then click the size you'd like.