Add the following CSS snippet into Appearance > Customizer > Custom CSS:


1. Change logo image max height:

.site-header .site-title a img {

max-height: 100px;


Adjust the value until you satisfy.


2. Change logo container's size:

@media (min-width: 768px) {

.site-header .site-title {

width: 20%;

}

.site-header .primary-navigation {

width: 80%;

}

}

Notice that sum of 2 value is 100%. Adjust the value until you satisfy.