That's a pretty simple pattern and I'm not sure that anyone can copyright a nine-patch because it's been around awhile. The size just depends on what you want to work with.
ie: if you cut your strips for the 9patch at 1 1/2" then your squares would be cut at 3 1/2"
2" strips = 5" sq
2 1/2" strips= 6 1/2" squares
and that's as big as I'd go, any bigger would look clumsy in my opinion.