As with aperture, bigger is generally better. If you're talking about a ready-made pier, then 6 to 8 inches in diameter will be plenty.
However, it also depends on the height of the mount above ground (length of the pier) and the weight you put on it. The 6-in achromat is long-ish, but not too long for a 6-inch pier if it's not loaded with accessories.
If you anticipate doing astrophotography and will have a guide scope, cameras, rings, etc., at some point, then an 8-inch pier would be better.
A simple thin-walled steel tube will suffice if it's not tapered to a point under the mount and if it's sufficiently anchored. I use standard 8-inch AstroPhysics piers of three different heights with my telescopes, and for the SV152 apo (which weighs a tad more than twice the CR-6) I use a 52-inch tall pier so I don't need to crouch at most eyepiece heights.
If you're using a solid concrete pad and setting the pier atop that, then there will be vibrations in the scope when you walk near the pier. It's best for a permanent observatory to pour the pier footing separately from the pad and to isolate the plinth from the rest of the pad (you can use sand for isolation) to prevent vibrations from transferring from the pad to the pier.