Dish uses 1 5/8" outside diameter
A 1 5/8" fence post works well for me, but if you really want to be future-proof you could put up a really sturdy, really big pole and then use a pole to pole mounting kit to attach the correct post size for your dish to the heavy duty, grounded, "master pole". Radidio Hut has the pole to pole mounting kits as does a place called Moyer Electronics. The kits are sometimes called a vent pipe mounting kit and they support up to a 5" OD master pole and pretty much any size dish pole up to 2.5" These are also good mounting kits if you already have a Big Ugly Dish planted in your back yard and you just want any easy way to add an additional small DSS dish.