Sizing: You can grill any size mushroom, but you need to use different equipment and methods to get different sized mushrooms cooked. Large portabella mushrooms (perfect as a hamburger patty can be grill as is, right on the grate. Any mushroom that will remain big enough not to slip through your cooking grate can simply be poured on the grill and cooking. Remember that mushrooms can lose as much as 50% of their size as they cook. Smaller mushrooms can simply be run onto skewers so they can be more easily controlled on the grill. Be gentle with mushrooms and use tongs for turning, not forks, or spatulas.
Preparing: Many people tell you that you shouldn’t wash your mushrooms in water. Mushrooms should be gently brushed of any remaining dirt or debris. Of course most restaurants, even the fancy ones, rinse off their mushrooms to clean them off. Individually brushing off you mushrooms should only be attempted if you have a lot of free time. As for the stems, just trim off the dry end, unless the stem is going to get in your way (like for a portabella mushroom burger). If you do rinse off the mushrooms before cooking do it right before they hit the grill.
Flavoring: You can marinated mushrooms in whatever you like for days in advance. Mushrooms are very forgiving that way and since they are going to be clean of bacteria you don’t have to worry about the marinade becoming contaminated. So you can marinate well in advance, but I find that mushrooms can be seasoned right on the grill. Take a small amount of olive oil, add garlic, salt and other seasonings and brush (or mop) this onto the mushrooms as you cook them. If you are preparing mushrooms as a side dish to something else season them with the same thing you have seasoned the main dish with. This gives you complimentary flavors.
