Stock bars, two bikes; 4 inch (Aeromach/LaMonster Honda pullbacks) and 5.5 inch risers (Harley Duece knockoffs).
This is not a looks issue (and on interstates, you can hardly see them), it is a comfort and ride-ability issue.
Stock risers are 3 1/8" rise.
When you lean too far forward riding (depending on your size), you can get arm, shoulder, back issues (shoulder is the main compliant). The longer the ride, the more noticeable the issues.
Sitting up at attention with feet down on the controls, it's not so bad. But kicked back on a driver's backrest, feet up on the Hwy pegs and going all day, it is an issue (for me anyway).
At 5'9", I like the 4" risers for performance twisty riding, but I like the 5.5s for the kicked back cruising riding.
This is old and has a couple small errors, but is good reference.
http://www.timskelton.com/valkyrie/how_to/risers.htm