Increase Menu Text Size in 'Clean'

Hi Clive,
“Could you explain how I increase the text size of the main menu in the ‘Clean’ WordPress theme?”
You can do customization via theme settings (customizer) or open custom.css file at theme_file/css/custom.css then add

.uk-navbar-nav > li > a {
    font-size: 16px;
}

“I would also like to increase the logo size and although I have found how to do this, the logo size is restricted by the size of the header area. Is there any way of this being changed?”
Could you please share your site to check ?
“Lastly, I would like to add contact details such as email address and telephone number above the header area (at the very top of the page). Can this be done and if so how?”
You can use custom html widget then assign to toolbar-l or toolbar-r widget positions.

Next time please ask the question via ticket support.
Thank you