It really does depend how you're cooking them.
Logically, smaller pieces would expose a greater surface area, however, I have experienced greater heat from a dish when using larger pieces.
The small pieces give an even heat that permeates the dish, but the big pieces give a burst of heat when you encounter them. Even using both wouldn't be unreasonable.
The difference will be reduced in long slow cooking, especially with some fat in the sauce, as the capsaicin (i.e. the heat) will have more chance of dissolving and spreading into the rest of the food.