Big header, scroll down, make header small (Like default size)

Support
4

You might be able to do something like that by putting the big logo in a custom header (admin>customize>css/html/new). Add a header to the ‘Header’ section of the editor:

<div class="wrap brand-header">
    <a href="<!-- your forum url -->">
        <img class="brand-logo" src="<!-- link to big logo -->">
    </a>
</div>

You could add some css to hide the regular sized forum logo until the custom header has scrolled out of view:

.brand-header .brand-logo {
    /* this moves it down a bit, this could be improved */
    position: relative;
    top: 25px;
    z-index: 1200;
}

/* hides the site logo when the custom header is in view */
.d-header #site-logo {
    display: none;
}

/* displays the site logo now that the custom header has scrolled out of view */
.docked .d-header #site-logo {
    display: inline;
}

This works, but is barely tested. Use at your own risk.