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 Eastern Rosellas is 65mm, they will also use entrances 70mm in diameter, but preference is not for any larger if there are smaller options. Crimson Rosellas can fit into entrances 70mm and above, so I would recommend making a for-Eastern Rosella nest box with an entrance of 65mm diameter. Also worth noting, Kookaburas can raid nest boxes with entrances 75mm and larger.
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.
