/www.aquarist-classifieds.co.uk/php/detail62_33316.phpDepends on the size filter you buy, I have the ones suitable upto 100litres in some of my fry tanks and the sponges are approx 7cm and the overall height of the base and sponge is about 11cm.
But if you ask Eddy from Aquarist he can give you the full range of the Huey's sizes -
http/www.aquarist-classifieds.co.uk/php/detail62_33316.php
I'm guessing its because the bigger the sponge, the more bacteria and surface area for bacteria to live on, as well as water to pass over, meaning more biological filtration.