for Crab, I dont like them small - too much shell.

Below 2lb I wont be bothered.

Fish is different - the ideal size for most fishes is just over a lb, some types can go up to 2lb - but only if you cook it by dumping it into boiling water, then immediately off fire/heat, and let the water temperature to continue "cook" the fish ... All Cantonese restaurants cook the "steamed" fish this way. At home, I would only pick 1lb fish for steaming.

Steam is the best way for fresh seafoods.