Through many years as a bull with many couples nearly all the guys I've cuckolded were 4-5 inches. Although I don't have measurements on all the guys and this is an estimate, I don't think any cuck was smaller than 4 inches.
Years ago, an ex gf described the guy she dated before me as being...
MY wife sucks the biggest cock of her life, after she sucked him dry, he wanted her pussy and then her ass. She was a trooper and took all he had to give.