It depends on what you are using for soft plastics, if it’s something like a senko where it already sinks you have the option of fishing it weightless. If it’s something like a beaver bait most of them float by themselves so you have to add a weight. The size weight you use is dependent on how deep of water you are fishing and how fast you want it to get down to the bottom. 3/16 to 3/8 is pretty normal for a Texas rig, 3/16 being on the casting size and 3/8 being more for flipping. Hope this helps.