The one that has the biggest knob might not fit my hand so that would make it difficult to hit someone with if I don’t have full control over it
Discussion
You want it to be appoximate to the length from your feet to your hip.
Yep, I was gonna say I’m also short
That's very true. Better to have it balanced. Strikes are very rapid with the Shillelagh, big lumbering heavy hitters aren't actually part of this style of stick fighting. The speed, accuracy and the law of the lever are what make it effective, the knob just concentrates it all that force onto a small area.
The size of your knob isn't everything ;)