Well I think the cylinders were 14 inch stroke . A 14 inch cylinder gave it more down float for uneven ground . But when raised they needed a cylinder stop put on . 
Really I just did some measuring with the marker up and down . Held the cylinder in place with it  up and down . I think I might have  got 12 inch cylinders at first and determined they would not allow the marker to go into low spots  . Other than the cylinder and hoses I used scrap steel from the pile  .After that I did a quick tack up of the mounting and manually lifted the marker up and down to check  the geometry . 
I do remember now that I did need to beef up the mounting eye on the rod end of the marker .
My original was just welded on with out any gusseting .

I may still have the markers . I took them off when I sold the planter . I can measure the cylinders . Dont hold me to those lengths .