You’ll find we do all sort of things as far as nesting boxes. It’s not a question of right or wrong, just a case of what we do.
A general rule of thumb is that they need to be at least 12” wide X 12” deep with an inside height of at least 12” for full-sized fowl, which is what you have. You’ll see photos of people using things that are not that big. You’ll see a lot that are a lot bigger, even going to “community” nesting boxes. I personally went with 16” x 16” x 16” which works well.
Not knowing anything about what you are doing, make then at least 12” in each dimension, with bigger not a problem.