Flycatcher chooses duck box for nest

Great Crested Fllycatchers have nested in our neighborhood for several years. I've put up two nest boxes of appropriate size, with no takers. Apparently, a larger size is preferred. The birds are nesting in our yard this summer, using a duck box vacated by Wood Ducks about six weeks ago. I see the flycatchers bringing nesting material to the box. I wonder how much will be enough.