Hi Zhenjie,
The width of the spreader is determined by the width of your torso. A narrow spreader like the 6" is best for the ladies and kids with narrow waists. A small spreader on a big waist makes the sides of the harness pull in too hard and will apply massive pressure to your lower ribs.
Too wide and you won't be able to do the harness up tight enough. The average would be 10 inch spreader, 12 inch for the bug guys.
Good winds,