Chassis/suspension tuning is half personal preference and half rule. The parts that are RULE...
really need to be listened to.. or else "your gonna have a bad time"
The
main point of swaybars is to reduce body-roll.
Pickups generally don't come with a rear bar because there isn't much weight over the rear.. so the vast majority of its body roll is going to come from the front end. This means that It's more or less a "waste" to put a rear bar on *most* pickups. However.. some folks usage of their trucks would absolutely benefit from a rear bar..
Towing for example... a rear bar will help alleviate some sway from the trailer... or If the bed is frequently loaded up with a meaningful amount of weight.. as a bunch of weight in the bed could/would cause body-roll.
Suvs & cars will usually always have a front and a rear bar because their weight distribution (and therefore body-roll) is closer to 50/50.
Diameter of either bar largely depends on the weight of the vehicle and its intended or desired purpose... but for user enjoyment/safety purposes its extremely important that both bars are sized to
work together.
Like i said previous... too big of a rear bar when compared to the one on the front can be straight up dangerous. The bigger the rear bar gets, the more prone to OVERsteer the vehicle will become. This is why everyone (during my days of subaru-dom) with a plain impreza or a wrx would want to upgrade to either a wrx or an STi rear bar... to make drifting/donuts a much easier process.
Conversely... the bigger the front bar gets... the vehicle will experience more UNDERsteer or "pushing" through a corner.
Swaybars really are a delicate balance. This is why I never mess with their settings in racing simulators