How to adjust screen size

check you have the correct resolution set--right click the desktop and select resolution and if possible increase it

if your resolution is ok right click the desktop then select personalise then desktop background--it has a dropdown box with fill -fit--stretch--tile-- center

it sounds like yours is on center--change it to fill
 
in xp: go to display properties -> set wallpaper to 'stretch'. if the wallpaper is smaller than your display resolution it might distort.
in windows 7: go to control panel > all control panel items > personalization > desktop back grounds > picture position > stretch > save changes.
or
click the start button > type wallpaper in the search box > click 'change desktop background > picture position > stretch > save changes.