No worries, I solved it. This is how you can adjust the size of the header:
.custom-header .site-branding {
background-color: transparent;
margin-top: 25px;
margin-bottom: 10px;
padding-right: 0;
padding-left: 0;
height: 180px;
width: auto;
}
And this is how you can adjust the width of the menu and widget area on the right:
.menu li {
margin-left: 50px;
width: 170px;
height: 40px;
font-size: 16px;
}
.side-widget-area {
padding-left: 50px;
width: 170px;
}
