If you have selected text and want to change its point size, you can easily do so with a simple keystroke: Ctrl+[ (the square bracket key next to the letter P on most keyboards) will lower the selected text by 1 point. Tap it a few times to see the effect. Similarly, Ctrl+] increases the text size.
You can also use Ctrl+< and Ctrl+> — but while that may be easier to remember, you have to use the Shift key, so I prefer the square brackets. Also, that keystroke doesn’t work in Outlook.
(If you need to, you can review how to select text with the keyboard.)
Excel doesn’t allow any of these keystrokes. Excel is a bit of a fuddy-duddy that way, refusing to play along.