For our wok recipes, we like to have our portabella mushrooms in big enough pieces such that we can appreciate them in their own right, but not so huge that they dominate the jumbo shrimp and other ingredients too much.
We'll generally slice our portabellas to be about 1/4 inch (< 1 cm) thick and perhaps up to 1 1/2 inches long (3-4 cm) which gives them a cooking time about the same as our fresh bell peppers and lets us add them as part of the first ingredients we put in the wok (long before the jumbo shrimp or jumbo prawns which come last) and have them be cooked to perfection when the fermented black beans, fresh garlic and fresh ginger root are completely cooked as well and have blended their flavors into the overall meal.
Portabella Mushroom Sliced and Ready to Wok
For more flavor, use older portabella mushrooms.