change header height on 2017 theme for logged in users

WordPress will automatically add a body class to the body tag if a user is logged in. You could potentially use this to adjust the height.

For example..

.logged-in #myElement {
    ...Your CSS
}

Something like this should work.

.logged-in.twentyseventeen-front-page.has-header-image .custom-header-media {
    height: calc(70vh);
}

Note the calc 70vh is 70% of viewport height, so adjust to what you need.