Fresh mushrooms dry at a rate of about 10 - 1. That is, it takes about 10 pounds of fresh mushrooms to make 1 pound of dry mushrooms. So an ounce of dry mushrooms is equal, in a way, to slightly over a half of pound of fresh. Because there is more flavor in dry mushrooms than fresh, they will have a bigger flavor influence than might be expected from their small volume.
(Source: wildaboutmushrooms.net)
(Source: wildaboutmushrooms.net)