How to customize height of Header Image

Hello, I'm wondering how to customize height of header image. I spend a lot of time on the _header.scss, but failed to find a right "height" to get it worked. Basically, my header image was automatically cropped after uploading so that it looks strange. I wanted to customize the height of header image so as to match with the picture i would like to use direclty. Please shed some light here, thanks so much! Lucien