Part of the fun is being able to use different sizes and experiment with something you've not done before.
Now if I am making one as I am a miniaturist and more comfortable with small I would be making mine less than 18" at least. And that size is more than I like.
But others are more comfortable with larger sizes like 35" - 40" or so. as long as it can be hung on a wall without being too huge to hang comfortably then you are fine.
I'm sure it takes trial and error to find what size you like to work with.
Also keep in mind not everyone has lots of wall space. But some also will use them as table toppers or put them on the back of a couch or chair etc. Just some things to think about when you consider the size you use.