Why has my header logo shrunk on desktop?

Hi all,
My header logo used to stretch all the way across the page (which is how I want it) in desktop. But for some reason, it’s now really tiny. I’ve tried changing the settings on the site but nothing works. It won’t let me make the logo any wider than 600px in the settings so is that the problem?
I’m a beginner, by the way.
(I have a different image set for tablet and mobile and these still seem fine)
Many thanks

The page I need help with: [log in to see the link]