Returning to the OP’s question... Let the size of your portable toilet be dictated by two factors in tension: (1) how long between dumps- bigger is better, and (2) how much you can carry and empty conveniently- smaller is better. Figure 8 pounds per gallon of capacity, since it will be mostly water, plus the weight
of the tank. That means a 5 gallon waste tank will weigh around 40-45# when full.
Comfort is also a factor favoring a larger unit- it’s less like squatting and more like using a conventional toilet. Some set a smaller toilet on a platform to make it easier to use.
While they can be emptied in a flush toilet, it takes care not to splash or overflow and make a mess, especially with a larger tank. Vault toilets are better, and dump stations the preferred means of disposal.
If the thought of being that close to human waste makes you queasy, you might want to reconsider a conventional RV toilet with black tank. Or, as many do, limit the portable toilet to liquids only.