Hi Stephan,

 it can be done with CSS media queries.

#logo img {
    height: 60px;
}

this is the normal logo size

@media only screen and (max-width: 767px) {

    #logo img { height: 60px; }

}

and this is the logo for devices with max screen width of 767px

of course change the size (60px) and add the code to  theme options -> styling options -> custom CSS