The Pixelate effect combines the pixels in the video image into larger squares and makes pixels in that square the same color, which is an average of the original colors of all the included pixels. It produces a blurring effect (but different from the actual Blur effect).
Pixel Size defines the number of neighboring pixels that this effect will group. The larger the size of each square, the fewer squares there will be in the picture, and the less detail will be left in the modified image.
Keep pixel size constant
Check this option if you want the size of square to remain constant throughout the duration of the effect. If this checkbox is not checked, the effect will start with the original image, and gradually increase the size of the squares, only reaching the specified size at the end of the effect's duration.