How to change the size / height of the logo in LiveEditor?

The logo width is set at 250px currently to help stop people breaking their pages with huge logos.

You can override this with a little CSS: Add this in the Page Settings > Other Scripts > Choose Custom CSS from the dropdown section (obviously you'll want to adjust the values to what you want)

 .banner .op-logo img {height: 100px !important;width:300px !important; max-width:300px !important}