Our friend Woody from Munson, MA makes the best nesting boxes ever.  This one is for chickadees only!  The hole size is 1 and 1/8 inches.  It’s smaller than a bluebird size box.  This is 12 inches high (including the back with a predrilled screw hole and comes with a screw), 5 inches wide, 5 inches deep.  To clean, just turn the front gold latch and the front panel opens.  Easy.  Made from long lasting cedar.  Make sure you clean out any crud from winter or last year in the spring.