I often end up with strange font sizing issues when composing an email where part of my email are in a larger font size than others. To change the font of your email, you can only increase or decrease the font size through the designated buttons or by changing the paragraph format altogether.
In my opinion it would be better to just display a size selection dropdown menu, just like pretty much every other application, allowing you to just pick the required font size and apply it to the selected text.