You really can use any size of zucchini to make zucchini bread; small, medium, or large. You need 2 cups of grated zucchini for the recipe, so you will need about 3 small zucchini, 2 medium zucchini, or 1 large zucchini. Most people like to use large, overgrown zucchini from their garden to make zucchini bread because they aren't as good to cook with but are great for baking and making zucchini bread. Never throw these big guys away, they are perfect for zucchini bread!