There is no option to do this on this template but you could add the following CSS to your custom.css file and I believe this should do it. You can adjust the scale to whatever number you desire 0.8, 0.5, etc
.s5_wrap_fmfullwidth #s5_logo_wrap img {
transform: scale(0.7);
-webkit-transform: scale(0.7);
-moz-transform: scale(0.7);
-ms-transform: scale(0.7);
-o-transform: scale(0.7);}