That error is caused by trying to write a frame that is a different size than the very first frame in the video. All frames in the video must be exactly the same size.
If you are using getframe() or saveas() or print() to capture information from the display, then you need to be aware that the size of what is displayed can change over time if you are not careful. To write a video frame it is recommend that you either generate your frame into a matrix of known size (not capturing from the display), or that if you do capture, that you imresize() or impad() or imcrop() to a fixed size.
