Yes I am aware that you are not talking about the header title but adding padding above and below the header title will create a larger area for the header image to fill.
I gave this answer because it has already been asked a few times on our support forum and this is the way we recommend to increase the height of the header image area as it also keeps the page title centered vertically.
Anyway, to apply it to the home page only use this instead:
.home .header-title {
padding: 70px 0;
}…or this if you prefer to set the height of the main header:
.home .main-header {
height: 700px;
}