Actually, idle jets can be partially active up to about 3500rpm depending on airflow of the motor.
I believe the stacks I have are 2.25 inch and that's what Webers usually come with. I would recommend a set, the 2.25 size fits under the air cleaner and work just fine.
I don't think the velocity stacks will alter your jetting much(especially short ones in a Speedster), but they are good for drivability.