Maximum height of hand-shaped concrete curb?
How much concrete slumps is directly related to how much water is used to mix it. Interestingly, (within reason) drier mixes are also stronger than wetter mixes, all else being equal. Most people mix too wet by default.

I'd firmly suggest some experimentation (by which I mean not "try to do a section" but to actually build a short test piece that you will discard.)

You may find this description of the standard "slump test" somewhat informative to your application:

https://en.wikipedia.org/wiki/Concrete_slump_test