Hello @wonderacademie,
Thanks for writing to us.
I checked your website it seems that you have already set site layout wide and full width.
You can customize it more as per your need from here.
Please follow these steps
Go to Dashboard --> X--Theme options-- Layout and Design
Please have a look at the given screenshot below. You can customize from here.
For your 2nd query
You can set your header Fixed at the top to achieve your desired layout as you have mentioned.
Go to Dashboard --> X--Theme options--Header
You will get more settings here, you need to select “Navbar Position” as “Fixed Top” to achieve your desired layout.
Please have a look at the given screenshot below

For your 3rd Query
It totally depends upon your content, logo size and your personal choice. You can customize any layout for the header as per your choice.
Thanks