Mushrooms are particularly well suited to grilling. Most are at least 80% water by weight, allowing for aggressive heat treatment. Gun for a mix of cultivated and foraged, if possible, for both visual and flavor complexity. Take a look at the surface area of the mushrooms, and make your best guess at tearing and cutting the various mushrooms into pieces that will allow for uniform cooking time. Don't be afraid to use plenty of white mushrooms. They are perfectly lovely, and do not warrant the "plain" reputation they often carry. Mushrooms are glorious carriers of all things bulby and herby, but they need to be mindfully seasoned--using ingredients both sweet and salty--or they run the risk of being one-dimensional. And, hey, wash your mushrooms before you use them. They absorb an insignificant amount of water.