Another interesting question: If you could design your own instruction set, what is the smallest you could make a complete chess program? Is there an ultimate limit, information-theory style?
(That's a long way to say that the question is somewhat under-constrained.)
Instead consider a CPU architecture and accompanying notation designed especially for playing chess. The other idea is not very interesting.
Assuming some arbitrary architecture plus the necessary code to emulate it on a normal architecture (including your stupid CHS example) would normalize the "size" of the program, anyway -- unless there are hardware structures that are especially good for chess. Which was the essence of the question.