There are several inserts for the carrier of the Opemus 6, and what you linked to is the 6x4,5 kind. You can modify the carrier you have by changing inserts (masks). So, if you want to print 6x6, then you'll have to get a 6x6 insert. You do this by removing two plates, one for each part of the carrier, which are held in place by two screws. That said, you also need to take into account that your negatives might "pop" in the enlarger when the lamp is on, because of the generated heat. So, having a glass at the top would probably be a good idea, comined with an insert at the bottom. The top glass looks "frosty", it's anti newton ring glass. Of course, having a glass carrier also means that you need to keep the glasses clean too. Any dirt - dust on them will show on the prints. Ideally, you'd like to have different carriers for any different format you use, so buying an extra carrier would be nice. Oh, and you also need a 80mm lens as others have said.