Acording to the staff at tropicorum, in uncured rock, you want to look for a piece with lots of green stuff and other signs of life on it, wich I believe you should also look for in cured rock (wich really isn't cured in my honest opinion, becuase of how it's shipped), but won't be as apparent in my expierience. Another thing to look for in a piece of live rock is that it's porous, this way it can suport more bacteria and provide a good in tank refugium for your pods. One thing to watch out for is aiptasia anemone, wich ARE a pest.
As for size and shape, I think it's best to look for variety in your rocks, as well as looking for rocks that you have a place for in your aquascape, so you don't have to redo the tank for one rock. When purchasing more than one rock, a good "variety" of rocks to get would be 6 bolders and 2 pieces of large flat rocks, this way you can have a sturdy, 2 tier cave system without a hassle.