You've not stated how you're viewing your email. (With an email client, such as Outlook Express, or via the web?).
However I'm assuming that double-clicking on the title of an email (in Outlook Express, or similar) is opening a window which is too small for your needs. I usually simply click on the 'maximise' (square) symbol which, as you state, makes the email window fill the whole of the screen. If that's too big for your needs, place your mouse cursor over the edges of the small window and, by holding down the left mouse key, drag the edges until the window has the size and position you require. Your computer should remember those details, so that other email windows will then open in the same way.
Chris