Dear Francois Zietsman,
To increase the logo height you need to customize header.php template keeping on child theme. You need to increase logo container class span4 to span6 and also need to change the menu container to span8 to span6. Take reference of below screenshot
Note: Please customize the header template only if you are technically sound on WordPress otherwise will get php fatal error. In this case we recommend you to take help from a developer.