I have only owned and used one heat pack. I used it many times for commercial golf ball diving in cold water and this was maybe 15 years ago. They don't throw out a tremendous amount of heat, so I think I would recommend the largest pack that you can comfortably wear in your suit.
Sorry I don't have more detailed or current information on these devices. They really can make the second or third tank of the day much more comfortable when your core temperature has already been lowered. Once this occurs, insulation is not really the solution.
You need to add an external heat source and/or increase your metabolism and heat generation by exercise and consumption of calorie rich food. I know exercise after a dive is not recommended due to decompression issues, but a few sets of 20 reps of deep knee bends that gets the large quad muscles working can begin to generate some heat, before the dive.