I always have to change the font when I send messages from my computer. I tried dragging the Smaller/Bigger text button on to the toolbar and it doesn't allow me to use it.. anyone else having this problem?
The buttons are grayed out on mine too, but you can go into Mail's preferences and change the font size under the "Fonts & Color" tab and it will stay that size, use the "Message font" box to change just the Compose mail font.