I agree with the above.
On a 5' tall ramp you are going to be eating up 4' or so of horizontal distance when you carve in or out of a grind. Assuming you only grind less than 4', you are still going to be close to falling off the edges. Either build lower or wider.
Also, 8' radius is pretty much your standard very mellow transition. 5.5' on a 5' ramp is not mellow at all. I would dig it but a beginner would not. You can do a tighter transition on a smaller ramp without it being as difficult or intimidating to ride but keep in mind that nearly everything looks steeper than your template once you build it and are standing on top. 5.5' radius on a 3' might be fun but the best idea might be to build a few quarter pipes out of wood and figure out what you like before its set in concrete forever.