resizing the logo in the header / expanding header

Hi there, I looked though all the forum and docs but could not find the appropriate answer – although I believe that this has been adressed somewhen.

I would like to have a bigger logo – I know that the size is initially limited. to resize the logo I thing the header needs to expand too (otherwise it woudld cut the logo off).
any hint where and how i can change the css to achieve that?

thanks in advance
P.-