The best size skillet to make Egg In A Hole with mushrooms and spinach is a 12-inch skillet. It fits everything perfectly, including the entire bag of spinach. If you have a smaller skillet, you may need to cook the spinach in batches. That's by no means the end of the world, though. So if you only have a smaller skillet, just use that. If you only have an 8-inch frying pan, I would suggest cooking in batches. Four servings may have a hard time fitting in an 8-inch pan. Cook all the mushrooms at once, then set half aside. Cook half the spinach with half the mushrooms, then make two wells and cook the eggs. Plate those two servings, then add the rest of the mushrooms and spinach to the pan, cook those together, add the eggs, and you're good to go.