You know... I had this problem, too. The only size my enlarger would enlarge to was around 8"x10" (that's like 20.3cm x 25.4cm). But I have a Beseler 23 II (old...blue...). I found I had to get a recessed lens board. Sure did the trick! Now I can make any size I want (and a few I don't)!
I have no idea what your enlarger is like as far as how the lens is mounted, but maybe this is a possibility. I bought mine on Ebay for a song!