I thought perhaps they ran the gamut (and they do), but the more I looked at various mid-18th century gowns, the more I noticed the same proportions.
Proportion in historical costume is just as important today as it was then. Proportioned garments are why that 1760s Robe a la Francaise looks itty-bitty, but actually has a 30 inch waist. Too wide in the panniers didn't seem to be a problem, but too narrow does not flatter the figure.
So what are these magical proportions?
Thirds and Quarters.
The 1, 2, 3, and 4 on the bottom are the quarters - these divide the hem of the skirt into four parts. These parts are roughly equal to the width of your shoulders.
You can use Thirds and Quarters to break down the width of the hoops in your pannier, too. For instance, my bottom hoop will be 2 thirds added to two quarters, like this:
So with these measurements, I know that my pannier, at the top, will be 39 inches across, and my bottom hoop 54 inches across. Multiply each of these by two, to account for front and back, and that will help determine the length of your pannier hoops. (You will need to add the depth to this front/back measurement, or else you pannier, when wrangled into hooped position, will "shrink")
Whether you are robust or thin, this proportion should work for you.
If your waist is 36 inches, then your grand pannier should be 54 inches across at the waist.
If your waist is 20 inches, then your grand pannier should be 30 inches across at the waist.
These are not hard and fast rules, just some things to consider when planning your side-hooped ensembles. Remember, bigger is always fine, but too narrow and you may not be displaying yourself or your gown in the best way.