casca,

I don't know the size per pound number for Tilapia, but let me give you this to think about.

I would prefer 2 four ounce Tilapia over 1 eight ounce Tilapia. Why? Because a 4 ounce Tilapia will begin spawning right away and 2 spawners are better than 1.

Now, if you are interested in growing them out for food, assuming you can catch them, then yes 8 ounce would be preferrable. I'm after forage fish and algae control and the more mouths out there the better.