Use CSS but have a G50 command to clamp the maximum spindle speed at what you feel comfortable with.
I am not aware of any guidelines for max rpm based on workpiec size, other than those related to the cutting speed.
For doing large diameters I have used two approaches to ensure a good grip:
One for manual machines is to tap the work and put a length of threaded rod through the spinlde with a flange and nut on the end to act as a draw bolt so the work is fimly pulled into the chuck.
The draw bolt idea is not workable on a CNC with a hydraulic chuck so I have machined custom step jaws with a recess toward the back and prepped the work with a groove so there was a mechanical interlock when the part was gripped. A variation on this which I have used is to machine the ID of the jaws with grooves that have a standard thread profile (but no lead) and mating grooves on the workpiece. This gives a very secure grip.
An open mind is a virtue...so long as all the common sense has not leaked out.