Change logo size

Hello,

I’m working with a child theme of your author theme. I was wondering if it is possible to change the size of the logo. I would like it to be smaller than the default theme size. I have been trying some css changes, but not with the desired result so far.
Can you tell me what selector I should use to change the logo size? Or do I have to look for a php function which declaires the size of the logo?

thanks in advance,
Bert.