At the top right corner you should see a dash/underlining sign like this _ a sqaure over a square and an X. Click the middle one(square over sqaure) and it shold re-size to full.
Sounds as though you need to increase your screen resolution. Right-click a blank area on your desktop and in the pop-up box select properties which will present you with a tabbed window. Select the settings tab which will reveal a slider to alter the screen resolution. Slide this to the right to increase the resolution until you arrive at a setting which suits.