I would think most muffler suppliers and manufacturers have catalogs that would be of help. They typically show length, width, shape, and inlet/outlet placements.
I suspect the Pypes muffler is simply matched to fit the car application as factory and makes it easier with a known muffler that works with whatever exhaust system that is chosen.
Just for fun, check out Summit. They have photos. These are all 3"
https://www.summitracing.com/search/...-in%3A3-000-in