I have Serama crossbreeds, and none has complained about her nestbox being too big. I have some lidded plastic storage bins that are about 12" by 12" by 14", a round lidded cat litter box that's about 14" in diameter, and an old drawer turned on its side that's about 10" by 14" by 14". They lay quite happily in all of these.
If you want to build or use something smaller, just take a look at the size of your hens, then build or look for something that's about 2 or 3 inches bigger than that on all sides. Say, 10 or 12 inches square? That ought to do it.