Guess you have to count how many pixels the area is and then calculate how big the checkers have to be.
In Photoshop I use this trick to find out how many pixels an area is: I make a selection of the area and copy it to the clopboard.
Then I start a new image. Photoshop suggests the size of the image on the clipboard (your selection) and that way you see how big the area is.
Another way is using the rulers but that leaves a bit more calculating![]()