You've not indicated whether you're using an email client (such as OutlooK Express, Windows Live Mail or Thunderbird) or sending your mail via a web-based service.
However, in either case, I'd expect ToraToaTora's advice to work. Either the box will get bigger or a scroll bar will appear down the side, allowing you to review the text throughout the mail.
However many systems also allow you to make the box bigger simply by placing your cursor over the bottom of it and then dragging it down the screen to where you want it to be (with your mouse's left-button held down).
It's much the same as when I'm typing my answer here now. I've used up more space than the box appears to allow for but a scroll bar has appeared down the right-hand-side of the box, so that I can carry on typing. If I'd have wanted the box bigger anyway, I could have clicked-&-dragged on the bottom right-hand corner of it.