Use the smaller charcoal ring from an 18/12 inch WSM, if you have one. It seems to provide sufficient heat for low & slow. As I am sure you already know, a bigger fire creates more heat. IMHO, the difference in the size of the smaller vs. larger WSM is not enough to cause that much difference in thermal dissipation.
If you don’t have a smaller charcoal ring, I would put something in the larger ring to take up some space. Like a fire brick or two. Something safe and able to withstand the fire. It should reduce the fuel level and provide some heat stability.
I would use the minion method and only start about 15 to 20 coals at the beginning. This will help with control ramping up to your target temp. You don’t want to overshoot your target temp as the WSMs are easier to heat up then cool down. Once the WSM has settled down in your target range, only make small adjustments with the bottom vents. Keep the top vent fully open at all times.
Also, I wouldn’t load the cooker up full during my first cook. I would only cook at half capacity for the first time to establish a reference point for future cooks.
I believe Captain Morgan has a large WSM. Maybe he can chime in with some advise from his experience.