This is how you can force OS X to use the proper resolution of your monitor.
Open up Finder, go to the Applications folder, then into Utilities, and open up Terminal.
Now, type the following command:
sudo nano /Library/Preferences/SystemConfiguration/com.apple.Boot.plist
You will then be prompted for the administrator password for the computer. Enter it and and press enter. This will open the console based text editing program named “nano”, with the administrative permissions (required so you can edit the file), and opens the configuration file for the system resolution. You should now see something like this screen:
If you don’t already have a line in here for the Graphics Mode, you can add it so that it looks like it does in the screenshot. After this line you’ll add a String element, with the WIDTHxHEIGHTx32 format inside of it. Once you’ve made the necessary changes, press Ctrl+O (Opt+O) to write the file and Ctrl+X (Opt+X) to exit the file. Once you do a reboot you should be good to go with the new resolution.
