the font size in the mail you are writing:
click on the little "TT" bitton that looks like that:
Chose the size of font you desire and its done.
For the font size of what you are viewing on your screen when you look your mails:
on a mac: press cmd (apple) and = (+) on the same time. to get it smaller again press cmd (apple and -
on a pc:
Explorer: Page, text size, Larger, largest, as your wish.