In Additional CSS you could write something like:
img.custom-logo {
max-width: none;
margin-bottom: -170px;
}
#masthead.scrolled img.custom-logo {
margin-bottom: 0;
}The second part is to stop the logo overlapping the page content when scrolling down the page.
Difficult to say exactly without knowing the dimensions and width/height ratio of your logo but you can always experiment with different values for margin-bottom and max-width if you find the logo is too large e.g. max-width: 200px;
Hope this helps.