How to change the size of the thumbnails in the pastelist

Hey,
thank you for making this amazing theme. I like it very much, but would prefer fullscreen scrolling blog posts (width and height). So my question is how can I change the size of the thumbnails in the postlist to fullscreen? Furthermore, I would like to adjust the size of the header-picture to 100% (height). Is there a possibility to chance this easily is the css file?
Thanks in advance,
Martina