Logo size

Hello guys,
I just edited the size of my logo with this code:

.brand.span3 {
    width: 15%;
}

Now is my problem that it’s displayed too small on mobile devices 🙁
I wanted to make the logo smaller to have more space for the menu and i don’t need space for the menu in the mobile version.
So is my question: Is there any way to keep the size of the logo as small as it is now but have the logo displayed in full screen on mobile devices?

Thanks 🙂