The largest ruler I have is 12.5 x 12.5. I like it. I also have the 6x24. I love being able to plop the 12x12 down and cutting a larger square or squaring a large block. My favorite HST method is two squares, sew 1/4 around the edges then cut in an X for 4 HST's. But it all depends upon how big is "big" for the child.