The metro system is super easy to use in Rome. It reminded me a lot of Bay Area Rapid Transport (BART). So I picked an inexpensive hotel near the Metro System, we took metro to the Trevi Fountain and Spanish Steps. I didn't like my Roman Hotel so I can't recommend it here.
There is a great clothing store near the Spanish Steps for plus sized women. The sale clerks could look at you and pick out your correct size.
I got a great skirt there. The store is called Pitran Donna here is the website. PITRAN taglie forti