[ls65536]

It's certainly possible, but remember that with 8 bits you have just 256 different possible representations, and every instruction opcode with its set of all distinct valid parameters will map to a number of those limited 256 states. For example, this makes it a big challenge to encode anything other than very small ranges of immediate values inside an instruction.

Typically whenever there's a reference to an N-bit instruction set, it's referring to the widths of the registers and data paths. Very often this is not the same as the instruction size, and in many cases instructions may even be of variable length.

So while it's much more restricting to commit yourself to only an 8-bit instruction size, it should still be possible to build something with it.