Do you mean that the image you're viewing is too large? Press x and it will resize to full screen. Pressing z will make it real size. There's more short cuts, check 'Help' in the menu bar:
Code:
+, =, [Keypad +] zoom in
-, [Keypad -] zoom out
Z, [Keypad /] zoom to original size
X, [Keypad *] zoom to fit within window
W zoom to fit width of window
H zoom to fit height of window
1, 2, 3, 4 zoom to Nx (3 = 3x)
9, 8, 7 zoom to -2, -3, -4, respectively
Cheers, Leon.