Well, now you can. It’s new! Here’s how to change your plain-text font:

• Click Options in the upper-right corner of your Mail page.
• Click General from the list on the left side.
• Find the words “Plain text font:” and choose a font and size you like from the pull-down menus. For reference, you’ll see a sample of the size and typeface to the right. The sample reads, “Sample Text.”