If you have a standard monitor (non-LCD) then there are generally controls
that allow you to expand the horizontal and vertical size of the image.
Unfortunately there's no easy way to describe where they may be on your
monitor since different vendors implement the controls differently. Check
your monitor manual.
If you have an LCD display it may be the you'll have to change the screen
resolution or make an adjustment in the system or video BIOS to allow
stretching. Again, you'll have to check your manual for the specifics.