There is a costumer who wants a fixed big 200px logo on top of their website.

I explained that this navbar is too big for screens that are not full HD. Why they think their big fixed logo on top of website matters so much?

Show them on an iPhone.

You know what, if you show them their own logo, they are going to love it because they're blinded by bias. Show them the same layout with a different company's logo, and ask them if they would leave. Guarantee the answer is yes.

Also, you're the designer.
It's your field, not theirs.
Patients don't tell surgeons how to perform surgery. Nor with architects, carpenters, plumbers, lawyers, chefs, musicians, actors, playwrights, mechanics, priests, etc.

You need a bigger logo so people know what site they're on.
-- actual argument in my office

@sylar182 These people need a bigger smack on their head to know which reality they're in.

@Farshad One way is to make it big on Full-HD, but shrink it relative to the window (not screen) size.

Especially if the logo is available as SVG, that's just a couple of CSS lines. They will only check the result on their big monitors, be happy, and it will still be OK for mobile.

Could suggest an auto-resize Logo, then show it to them on a big enough screen so its 200px..

Who the fuck still uses px ?
Ok SOMETIMES it can be ok (especially because it's faster to render)
But in general, just don't