What size prong collar would I need for an adult Rottweiler? 

I would suggest a medium or large link size, depending on the size of the Rottweiler. You will likely have to add or remove links for proper fit so a neck measurement will be needed.

We offer a number of collars based on your needs and budget. Stainless will be more expensive than chrome but both collars operate the same way.

This page will give you the info on measuring and fitting the collar.