My header logo has exactly the size recommended in Avia theme options. My header type is “Fixed header with social icons and additional …”. The logo is oversized even in the initial view with the “full height header” and stays at that size, when scrolling down & the header height is being reduced.
Had the same issue, found I had to reduce the logo to 90px tall to stop it from disappearing under the content area of the page in IE 8. It also does not “reduce” when responsiveness happens, but it’s liveable. The later IE versions all seemed to work fine.