Andrew on "How to change the size of my website menu bar?"

Your menu items have a bottom margin of 10 pixels:

1. Open your website in Chrome;
2. Right click on a menu item;
3. Left click on "Inspect element";
4. A new toolbar appears. Hover your mouse over the <li> element.
   Orange represents margin and green padding;
5. Click on the <li> element and look to the right of the toolbar;
6. In the right hand column scroll down until you see this style:
   

   .main-navigation .nav > li {
       float: none;
       display: inline-block;
       margin-bottom: 10px;
       zoom: 1;
   }

Override that in your Child Theme style.css file;

.main-navigation .nav > li {
    margin-bottom: 0;
}

Edit: You might want to apply that to desktop only.