Each sidebar has a unique ID and you can control the height using CSS:
.custom-sidebar-7 {
min-height: 400px;
}
The scroll speed of the parallax element is 25% slower than the scroll speed of the page so you have to make sure the height of the image is 25% larger than the height of the sidebar.
If it’s a full-width element you might want to check different viewport sizes to make sure the image doesn’t cut off during scrolling.