It depends where you plan to mount it. In my view 12 or 13 inches or more would be fine if you plan to mount it like a standard helm on say the outside of the bulkhead wall that separates the cabin from the fishing deck and use it more like the main helm. I have seen some that have a whole second helm station in that location including throttle, electronics etc. But, if you plan to mount it face up on the transom like mine, you want it to be smaller. Without going out and measuring it, I think mine is around 7 inches and has the knob.
There are a number of advantages to the smaller wheel:
It does not get in the way and is not a lot wider than the transom.
It is a lot more responsive in that it takes less arch/movement to make a course change or correction. This makes it easy to reach down and use quickly with one hand if you were say playing a fish and holding a rod. It’s hydraulic, you don’t need the mechanical advantage of the larger wheel to turn it as you would for the cheaper and more problematic cable systems which seem to fairly frequently get stiff or break strands and need cable replacement if constantly exposed to the marine environment.
I have a sloping canvas from the roof to the transom that covers the fishing deck when the boat is not in use and has a small pocket which goes over the small wheel and would be much more problematic to design to fit with a larger wheel.
The small wheel is cast aluminum and very heavy duty which makes it far easier to break free of the helm shaft with a good three arm gear puller without destroying the wheel or damaging the helm. When I had to replace a helm seal it sounded like a rifle shot when the gear puller broke the wheel free of the shaft.
If you want to add a third rigger off the transom there is more room for it with the smaller wheel and more room for additional rod holder mounts.