The different Rosella species are all generally a little different in size. Crimson Rosellas are generally more dominant and will out-compete smaller more timid Rosellas such as Eastern Rosellas. So it really is a good thing to ensure that you install nest boxes with slightly different entrance dimensions if you have more than one Rosella species in your area.
Also of consideration, is that species such as Kookaburras, Currawongs, Ravens and Brushtail possums will raid Rosella nests if they can access them. Thus a just-big-enough entrance size is vital to keep Rosella occupants safe from predators.
The just-big-enough entrance size for Crimson Rosellas is 70mm, they also really like entrances 75mm in diameter. However if a Kookaburra discovers the snack box, they can just reach inside to grab any eggs and chicks.
Here is a diagram with key design features for all nest boxes with a few special exceptions.
Here is some info on the different materials to use, or not to use when making your nest box.
