Those are a quarter size grate holes. So between 1/4 to 1/2 inch
That's awesome, you use the bigger grates. Now I know more.The plug it's on is 1 inch in diameter on top. So about .75-1"
Actually eggcrate is 5/8", not 1/4"
Also, determining size from eggcrate in the background can be deceptive because of field of view. Like hold a fish out in front of you for a picture makes the fish look larger than if you held the fish closer to your body. Generally speaking, a clear top down photo with the adjacent egg crate in plain view would make a good reference for size.
The favia frag in question is about .75-1"