Boot screen has wrong display resolution

I had the same issue, but resetting the PRAM solved it.

Procedure from here:

Resetting NVRAM / PRAM

1. Shut down your Mac.
2. Locate the following keys on the keyboard: Command (⌘), Option, P, and R.
3. You will need to hold these keys down simultaneously in step 4.
4. Turn on the computer.
5. Press and hold the Command-Option-P-R keys before the gray screen appears.
6. Hold the keys down until the computer restarts and you hear the startup sound for the second time.
7. Release the keys.

The problem shows after using an external monitor with a different resolution. For example a tv. Befor disconnecting the monitor set up the parameters foryour screen and then take the HDMI cable out.

Just tried out the solution to reconnect to my TV and go to system Preferences -> Displays and select Built-in Retina Display of the drop-down menu Optimize for :

Then take the HDMI cable out. It works!!