I'm double-checking something in my story to make sure it's physiologically possible. I have a race of humanoid beings in my story who, because their planet is 90% covered in water, have both lungs and gills, and spend just as much time underwater as they do on land. Now, for a being the size and shape of a human, how big would their gills have to be in order for them to...well...not die? I do realize fish are cold-blooded, but I figure, through evolution, these amphibious beings would have developed a warm-blooded version. (This is why I like sci-fi. When you write yourself into a corner, you can get yourself out of it by just making shit up ;)) So, how big should their gills be?