I went with the 11.5" standards to lower it an inch as well as increase ride
control and cornering stability. I have a 30" inseam and didn't want to give up any padding to drop it down. 
I had to adjust them just past the second notch to get the stiffness to
ridability ratio correct. Not sure if there is much room to go farther for
a passenger (and I don't care as I've got a piece of hard cold Rivco steel
back there anyway).
I really love the set up overall but anyone with longer legs may not like this length as your knees would be bent too much. 
If they blew out today I would probably replace them with 12" as those
would probably be perfect for my weight out-of-the-box and really
only add a 1/4" in height from where I ended up.
Perhaps a shock pilot fitment guide is called for?
Draft V1.0:
Height          Inseam          Weight           Progressive Shock Length
=============================================
50-60"          <25"              any               N/A Consider Rocket Trike
50-60"          25-30"            0-200lb          11"
50-60"          25-30"            200-500lb       11.5"
61-65"          25-30"            0-150lb          11" 
61-65"          25-30"            150-275lb       11.5" or 12" one up, 12" two up
61-65"          25-30"            275-500lb       12" one up, 12.5" two up*
66-72"          31-36"            0-150lb          12"
66-72"          31-36"            150-275lb       12.5" one up, 12.5" two up
66-72"          31-36"            275-500lb       12.5" one up, 13" two up*
73-79"          35-40"            0-150lb          12.5"
73-79"          35-40"            150-275lb       12.5" one up, 12.5" two up
73-79"          35-40"            275-500lb       12.5" one up, 13" two up*
* Heavy Duty if passenger is of like proportion.    
This of course is just a first pass and would some refinements and conversions to centimeters and kilos as appropriate....