Logo behavior in responsive layout

I'd like to replace the diamond logo in the Theme 1 default responsive layout in RH11 with an appropriately sized logo for my client. In the default layout, the logo retains its size relative to any text in the title bar as the browser is resized, and the logo disappears completely at the smaller smartphone size. When I use a different logo, or even modify the diamond logo, it is very small and gets even smaller at the smaller layout sizes. What is the secret to preventing the logo from resizing? Where in the .css is this controlled?

Carol