1/4" or 3/8" is fine.
Six standoffs is plenty.
Personally, I appreciate when there's a center standoff. If the installer is by him/her self, they can easily measure and determine where on the wall that top center anchor needs to be. Install that anchor, lift and hang the
sign on that, level the panel and mark your other holes.
A better option is a cardboard or Coroplast template. It's a whole lot easier than trying to hold a heavy panel in place while marking the holes.