It's fortunately quite easy to adjust the resolution your MacBook Pro sends over to your external display. Try this:
This should adjust your resolution to the maximum. To make these settings easier to get to in the future, check the box in the display preferences labeled "Show displays in menu bar". You'll see a small icon, meant to look like a display, appear in your menu bar. Going forward, you can click on this to set/change resolution settings without going through all the above steps.
- Click the blue Apple icon, at the top-left of your display (while connected to your 23" external monitor).
- Click on "System Preferences".
- Click on "Displays" in the "Hardware" row.
- A display preferences window should now open on each display... one on your laptop, and one on the 23".
- In the "Display" tab of this preference window, specifically the one located on your 23" display, should be a list of resolutions available. Scroll down and select the highest resolution, which for a 23" should be 1920x1200.
Hope this helps!