Change Header Height Canvas Theme

Hello, apologies if this has already been covered, been searching for days and I can’t seem to find the answer…

I’m slightly new to wordpress but am quite tech savy so not too bad with these things. I am trying to change my header height on my website to 100px high to shrink down the unwanted space. Everything I do though doesn’t seem to work. My website is http://www.stevemeadproductions.co.uk

Any help or hints would be greatly appreciated!

Thanks, Steve