Ok, to change the shrinking effect, you can adjust the padding-top on line 1201 in your style.css –
.site-header.light {
background-color: #000;
padding-top: 0;
}
The padding-top reduces to 0 which is what is removing the space from the top. If you prefer to have the space removed up top and when you scroll down, then you can change the padding to 0 on line 1196 –
.front-page .site-header {
border-bottom: 1px solid #ddd;
padding-top: 30px;
}
To set the logo size smaller, change the height to 56px on line 1255 –
.header-image .site-title > a {
background-size: contain !important;
display: block;
height: 56px;
text-indent: -9999px;
}
That will make the logo stay at 56px height both at the top and when you scroll down (when the shrinking effect kicks in).
Let me know if that helps.