You will need to adjust your screen resolution to fit your new monitor size.
If you use windows, click [Start][Control Panel] and then [Display Settings] or similar (note, if you have an NVidia card, this may be called NVidia Control Panel). If you can't find this, right-click your desktop and select properties. Depending on the version of windows you use, you may need to look through the different tabs, but you want an option called Screen Resolution or Size - it will be set to an option like 1024x768. Play around with different settings to see what looks best on your new monitor - if it is widescreen you may need something like 1280x800. When you hit apply, it will show you your new screen for 10 or 15 seconds, if it is ok then click ok, otherwise it will revert to the previous settings when the timer counts down, so if your screen goes very wrong, don't worry - wait a short while and it will return to the previous settings.
If you use linux then it depends on the distro you use, in Ubuntu it is under [System][Preferences][Screen Resolution]

 

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