Keep in mind that boa proportions are different from ball pythons- boas have a much longer body, so a 5-6 ft boa would be much thinner and have a smaller head than a 5-6 ft ball python. It would be a smaller snake.
Just like how a 5-6 foot blood python would be a large, thick snake.
Look at weight and not length if you just want something that feels girthy and sizeable.