It automatically assumes the paper is a standard size. I have the 7690 which is the model before it - see if this will work.
Go to print and then Printer Properties/Preferences (depending on what program you are working from)
Go to where it says custom under Size: and click - type in 8x8 and hit save
Now open the Size drop down menu and choose Custom 1. You should then see the changes reflected - see the attached screen shots.
HTH
