I use an ESU Visaton 41mm X 71mm rectangular speaker that fits in the fuel tank of all of my Atlas locomotives.
It's the same size as the factory installed speaker with the same impedance (8 ohms) and output (2 watts).
I install a 2nd speaker, TDS (formerly QSI) High Bass, that is 28mm diameter. It's also rated at 8 ohms and 2 watts.
Adding a 2nd speaker not only increases volume significantly but gives a richer, fuller sound. It's definitely the way to go.